Fix out-of-bounds panic in progress printer (#378)

* Fix out-of-bounds panic in progress printer

We ran into an out-of-bounds panic when we had more tasks reporting as
"currently running" than the number of status bars we had.

That was due to a lack of defensive checks in the face of a possible
race condition.

The race condition leads the `PrintStatuses` method seeing more
"currently running" tasks than there can technically be.

The race condition is between the `PrintStatus` method that prints the
slices of pointers to `*TaskStatus` and the tasks themselves updating
the status non-atomatically.

At least that's what it seems like. I'm not 100% sure about the cause
yet, but I could reproduce the issue by simply injecting more statuses
at the top of `PrintStatuses`. The code here fixes it.

* Add changelog entry

Author: mrnugget

CHANGELOG.md

@@ -17,14 +17,15 @@ All notable changes to `src-cli` are documented in this file.
 
 ### Fixed
 
+- Fixed a bug that could cause `src campaign [apply|preview]` to crash in rare circumstances when executing a campaign spec due to a bug in the logic for the progress bar. [#378](https://github.com/sourcegraph/src-cli/pull/378)
+
 ### Removed
 
 ## 3.21.9
 
 ### Added
 
 - Commands for campaigns no longer require the `-namespace` parameter. If omitted, campaigns will use the currently authenticated user as the namespace. [#372](https://github.com/sourcegraph/src-cli/pull/372)
-
 - `src campaign [apply|preview]` now caches the result of running steps in a repository even if they didn't produce changes.
 
 ## 3.21.8

cmd/src/campaign_progress_printer.go

@@ -37,7 +37,7 @@ type campaignProgressPrinter struct {
 	statusBarRepo map[int]string
 }
 
-func (p *campaignProgressPrinter) initProgressBar(statuses []*campaigns.TaskStatus) {
+func (p *campaignProgressPrinter) initProgressBar(statuses []*campaigns.TaskStatus) int {
 	numStatusBars := p.numParallelism
 	if len(statuses) < numStatusBars {
 		numStatusBars = len(statuses)
@@ -52,6 +52,8 @@ func (p *campaignProgressPrinter) initProgressBar(statuses []*campaigns.TaskStat
 		Label: fmt.Sprintf("Executing steps in %d repositories", len(statuses)),
 		Max:   float64(len(statuses)),
 	}}, statusBars, nil)
+
+	return numStatusBars
 }
 
 func (p *campaignProgressPrinter) Complete() {
@@ -65,8 +67,9 @@ func (p *campaignProgressPrinter) PrintStatuses(statuses []*campaigns.TaskStatus
 		return
 	}
 
+	var numStatusBars int
 	if p.progress == nil {
-		p.initProgressBar(statuses)
+		numStatusBars = p.initProgressBar(statuses)
 	}
 
 	newlyCompleted := []*campaigns.TaskStatus{}
@@ -110,13 +113,21 @@ func (p *campaignProgressPrinter) PrintStatuses(statuses []*campaigns.TaskStatus
 		newlyStarted[ts.RepoName] = ts
 		p.runningTasks[ts.RepoName] = ts
 
-		// Find free slot
+		// Find free status bar slot
 		_, ok := p.statusBarRepo[statusBarIndex]
 		for ok {
 			statusBarIndex += 1
 			_, ok = p.statusBarRepo[statusBarIndex]
 		}
 
+		if statusBarIndex >= numStatusBars {
+			// If the only free slot is past the number of status bars we
+			// have, there's a race condition going on where we have more tasks
+			// reporting as "currently executing" than could be executing, most
+			// likely because one of them hasn't been updated yet.
+			break
+		}
+
 		p.statusBarRepo[statusBarIndex] = ts.RepoName
 		p.repoStatusBar[ts.RepoName] = statusBarIndex
 	}