mirror of
https://github.com/letic/terraform-provider-google.git
synced 2024-09-07 09:36:44 +00:00
446 lines
12 KiB
Go
446 lines
12 KiB
Go
package dag
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import (
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"errors"
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"fmt"
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"log"
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"sync"
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"time"
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"github.com/hashicorp/go-multierror"
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)
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// Walker is used to walk every vertex of a graph in parallel.
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//
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// A vertex will only be walked when the dependencies of that vertex have
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// been walked. If two vertices can be walked at the same time, they will be.
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//
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// Update can be called to update the graph. This can be called even during
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// a walk, cahnging vertices/edges mid-walk. This should be done carefully.
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// If a vertex is removed but has already been executed, the result of that
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// execution (any error) is still returned by Wait. Changing or re-adding
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// a vertex that has already executed has no effect. Changing edges of
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// a vertex that has already executed has no effect.
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//
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// Non-parallelism can be enforced by introducing a lock in your callback
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// function. However, the goroutine overhead of a walk will remain.
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// Walker will create V*2 goroutines (one for each vertex, and dependency
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// waiter for each vertex). In general this should be of no concern unless
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// there are a huge number of vertices.
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//
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// The walk is depth first by default. This can be changed with the Reverse
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// option.
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//
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// A single walker is only valid for one graph walk. After the walk is complete
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// you must construct a new walker to walk again. State for the walk is never
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// deleted in case vertices or edges are changed.
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type Walker struct {
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// Callback is what is called for each vertex
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Callback WalkFunc
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// Reverse, if true, causes the source of an edge to depend on a target.
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// When false (default), the target depends on the source.
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Reverse bool
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// changeLock must be held to modify any of the fields below. Only Update
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// should modify these fields. Modifying them outside of Update can cause
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// serious problems.
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changeLock sync.Mutex
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vertices Set
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edges Set
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vertexMap map[Vertex]*walkerVertex
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// wait is done when all vertices have executed. It may become "undone"
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// if new vertices are added.
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wait sync.WaitGroup
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// errMap contains the errors recorded so far for execution. Reading
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// and writing should hold errLock.
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errMap map[Vertex]error
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errLock sync.Mutex
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}
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type walkerVertex struct {
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// These should only be set once on initialization and never written again.
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// They are not protected by a lock since they don't need to be since
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// they are write-once.
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// DoneCh is closed when this vertex has completed execution, regardless
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// of success.
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//
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// CancelCh is closed when the vertex should cancel execution. If execution
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// is already complete (DoneCh is closed), this has no effect. Otherwise,
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// execution is cancelled as quickly as possible.
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DoneCh chan struct{}
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CancelCh chan struct{}
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// Dependency information. Any changes to any of these fields requires
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// holding DepsLock.
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//
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// DepsCh is sent a single value that denotes whether the upstream deps
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// were successful (no errors). Any value sent means that the upstream
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// dependencies are complete. No other values will ever be sent again.
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//
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// DepsUpdateCh is closed when there is a new DepsCh set.
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DepsCh chan bool
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DepsUpdateCh chan struct{}
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DepsLock sync.Mutex
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// Below is not safe to read/write in parallel. This behavior is
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// enforced by changes only happening in Update. Nothing else should
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// ever modify these.
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deps map[Vertex]chan struct{}
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depsCancelCh chan struct{}
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}
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// errWalkUpstream is used in the errMap of a walk to note that an upstream
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// dependency failed so this vertex wasn't run. This is not shown in the final
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// user-returned error.
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var errWalkUpstream = errors.New("upstream dependency failed")
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// Wait waits for the completion of the walk and returns any errors (
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// in the form of a multierror) that occurred. Update should be called
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// to populate the walk with vertices and edges prior to calling this.
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//
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// Wait will return as soon as all currently known vertices are complete.
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// If you plan on calling Update with more vertices in the future, you
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// should not call Wait until after this is done.
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func (w *Walker) Wait() error {
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// Wait for completion
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w.wait.Wait()
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// Grab the error lock
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w.errLock.Lock()
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defer w.errLock.Unlock()
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// Build the error
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var result error
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for v, err := range w.errMap {
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if err != nil && err != errWalkUpstream {
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result = multierror.Append(result, fmt.Errorf(
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"%s: %s", VertexName(v), err))
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}
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}
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return result
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}
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// Update updates the currently executing walk with the given graph.
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// This will perform a diff of the vertices and edges and update the walker.
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// Already completed vertices remain completed (including any errors during
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// their execution).
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//
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// This returns immediately once the walker is updated; it does not wait
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// for completion of the walk.
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//
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// Multiple Updates can be called in parallel. Update can be called at any
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// time during a walk.
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func (w *Walker) Update(g *AcyclicGraph) {
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var v, e *Set
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if g != nil {
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v, e = g.vertices, g.edges
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}
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// Grab the change lock so no more updates happen but also so that
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// no new vertices are executed during this time since we may be
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// removing them.
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w.changeLock.Lock()
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defer w.changeLock.Unlock()
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// Initialize fields
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if w.vertexMap == nil {
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w.vertexMap = make(map[Vertex]*walkerVertex)
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}
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// Calculate all our sets
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newEdges := e.Difference(&w.edges)
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oldEdges := w.edges.Difference(e)
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newVerts := v.Difference(&w.vertices)
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oldVerts := w.vertices.Difference(v)
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// Add the new vertices
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for _, raw := range newVerts.List() {
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v := raw.(Vertex)
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// Add to the waitgroup so our walk is not done until everything finishes
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w.wait.Add(1)
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// Add to our own set so we know about it already
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log.Printf("[TRACE] dag/walk: added new vertex: %q", VertexName(v))
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w.vertices.Add(raw)
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// Initialize the vertex info
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info := &walkerVertex{
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DoneCh: make(chan struct{}),
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CancelCh: make(chan struct{}),
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deps: make(map[Vertex]chan struct{}),
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}
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// Add it to the map and kick off the walk
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w.vertexMap[v] = info
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}
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// Remove the old vertices
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for _, raw := range oldVerts.List() {
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v := raw.(Vertex)
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// Get the vertex info so we can cancel it
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info, ok := w.vertexMap[v]
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if !ok {
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// This vertex for some reason was never in our map. This
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// shouldn't be possible.
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continue
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}
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// Cancel the vertex
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close(info.CancelCh)
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// Delete it out of the map
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delete(w.vertexMap, v)
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log.Printf("[TRACE] dag/walk: removed vertex: %q", VertexName(v))
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w.vertices.Delete(raw)
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}
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// Add the new edges
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var changedDeps Set
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for _, raw := range newEdges.List() {
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edge := raw.(Edge)
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waiter, dep := w.edgeParts(edge)
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// Get the info for the waiter
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waiterInfo, ok := w.vertexMap[waiter]
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if !ok {
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// Vertex doesn't exist... shouldn't be possible but ignore.
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continue
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}
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// Get the info for the dep
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depInfo, ok := w.vertexMap[dep]
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if !ok {
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// Vertex doesn't exist... shouldn't be possible but ignore.
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continue
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}
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// Add the dependency to our waiter
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waiterInfo.deps[dep] = depInfo.DoneCh
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// Record that the deps changed for this waiter
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changedDeps.Add(waiter)
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log.Printf(
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"[TRACE] dag/walk: added edge: %q waiting on %q",
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VertexName(waiter), VertexName(dep))
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w.edges.Add(raw)
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}
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// Process reoved edges
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for _, raw := range oldEdges.List() {
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edge := raw.(Edge)
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waiter, dep := w.edgeParts(edge)
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// Get the info for the waiter
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waiterInfo, ok := w.vertexMap[waiter]
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if !ok {
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// Vertex doesn't exist... shouldn't be possible but ignore.
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continue
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}
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// Delete the dependency from the waiter
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delete(waiterInfo.deps, dep)
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// Record that the deps changed for this waiter
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changedDeps.Add(waiter)
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log.Printf(
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"[TRACE] dag/walk: removed edge: %q waiting on %q",
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VertexName(waiter), VertexName(dep))
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w.edges.Delete(raw)
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}
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// For each vertex with changed dependencies, we need to kick off
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// a new waiter and notify the vertex of the changes.
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for _, raw := range changedDeps.List() {
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v := raw.(Vertex)
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info, ok := w.vertexMap[v]
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if !ok {
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// Vertex doesn't exist... shouldn't be possible but ignore.
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continue
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}
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// Create a new done channel
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doneCh := make(chan bool, 1)
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// Create the channel we close for cancellation
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cancelCh := make(chan struct{})
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// Build a new deps copy
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deps := make(map[Vertex]<-chan struct{})
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for k, v := range info.deps {
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deps[k] = v
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}
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// Update the update channel
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info.DepsLock.Lock()
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if info.DepsUpdateCh != nil {
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close(info.DepsUpdateCh)
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}
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info.DepsCh = doneCh
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info.DepsUpdateCh = make(chan struct{})
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info.DepsLock.Unlock()
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// Cancel the older waiter
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if info.depsCancelCh != nil {
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close(info.depsCancelCh)
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}
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info.depsCancelCh = cancelCh
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log.Printf(
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"[TRACE] dag/walk: dependencies changed for %q, sending new deps",
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VertexName(v))
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// Start the waiter
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go w.waitDeps(v, deps, doneCh, cancelCh)
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}
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// Start all the new vertices. We do this at the end so that all
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// the edge waiters and changes are setup above.
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for _, raw := range newVerts.List() {
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v := raw.(Vertex)
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go w.walkVertex(v, w.vertexMap[v])
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}
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}
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// edgeParts returns the waiter and the dependency, in that order.
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// The waiter is waiting on the dependency.
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func (w *Walker) edgeParts(e Edge) (Vertex, Vertex) {
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if w.Reverse {
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return e.Source(), e.Target()
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}
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return e.Target(), e.Source()
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}
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// walkVertex walks a single vertex, waiting for any dependencies before
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// executing the callback.
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func (w *Walker) walkVertex(v Vertex, info *walkerVertex) {
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// When we're done executing, lower the waitgroup count
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defer w.wait.Done()
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// When we're done, always close our done channel
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defer close(info.DoneCh)
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// Wait for our dependencies. We create a [closed] deps channel so
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// that we can immediately fall through to load our actual DepsCh.
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var depsSuccess bool
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var depsUpdateCh chan struct{}
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depsCh := make(chan bool, 1)
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depsCh <- true
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close(depsCh)
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for {
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select {
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case <-info.CancelCh:
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// Cancel
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return
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case depsSuccess = <-depsCh:
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// Deps complete! Mark as nil to trigger completion handling.
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depsCh = nil
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case <-depsUpdateCh:
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// New deps, reloop
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}
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// Check if we have updated dependencies. This can happen if the
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// dependencies were satisfied exactly prior to an Update occurring.
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// In that case, we'd like to take into account new dependencies
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// if possible.
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info.DepsLock.Lock()
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if info.DepsCh != nil {
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depsCh = info.DepsCh
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info.DepsCh = nil
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}
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if info.DepsUpdateCh != nil {
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depsUpdateCh = info.DepsUpdateCh
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}
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info.DepsLock.Unlock()
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// If we still have no deps channel set, then we're done!
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if depsCh == nil {
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break
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}
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}
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// If we passed dependencies, we just want to check once more that
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// we're not cancelled, since this can happen just as dependencies pass.
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select {
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case <-info.CancelCh:
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// Cancelled during an update while dependencies completed.
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return
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default:
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}
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// Run our callback or note that our upstream failed
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var err error
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if depsSuccess {
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log.Printf("[TRACE] dag/walk: walking %q", VertexName(v))
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err = w.Callback(v)
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} else {
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log.Printf("[TRACE] dag/walk: upstream errored, not walking %q", VertexName(v))
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err = errWalkUpstream
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}
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// Record the error
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if err != nil {
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w.errLock.Lock()
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defer w.errLock.Unlock()
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if w.errMap == nil {
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w.errMap = make(map[Vertex]error)
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}
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w.errMap[v] = err
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}
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}
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func (w *Walker) waitDeps(
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v Vertex,
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deps map[Vertex]<-chan struct{},
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doneCh chan<- bool,
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cancelCh <-chan struct{}) {
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// For each dependency given to us, wait for it to complete
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for dep, depCh := range deps {
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DepSatisfied:
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for {
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select {
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case <-depCh:
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// Dependency satisfied!
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break DepSatisfied
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case <-cancelCh:
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// Wait cancelled. Note that we didn't satisfy dependencies
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// so that anything waiting on us also doesn't run.
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doneCh <- false
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return
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case <-time.After(time.Second * 5):
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log.Printf("[TRACE] dag/walk: vertex %q, waiting for: %q",
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VertexName(v), VertexName(dep))
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}
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}
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}
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// Dependencies satisfied! We need to check if any errored
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w.errLock.Lock()
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defer w.errLock.Unlock()
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for dep, _ := range deps {
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if w.errMap[dep] != nil {
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// One of our dependencies failed, so return false
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doneCh <- false
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return
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}
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}
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// All dependencies satisfied and successful
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doneCh <- true
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}
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