cmd/go/internal/work: decrement concurrentProcesses when action finishes

This fixes a bug where we only incremented concurrentProcesses but never
decremented it, causing us to run out of tokens and give all compiles
-c=1 after a point.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64_c2s16-perf_vs_parent,gotip-linux-amd64_c3h88-perf_vs_parent,gotip-linux-arm64_c4ah72-perf_vs_parent,gotip-linux-arm64_c4as16-perf_vs_parent
Change-Id: I41f4c1edb77004cbc1772d6d672045946a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/734260
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
TryBot-Bypass: Michael Matloob <matloob@google.com>

src/cmd/go/internal/work/exec.go

@@ -248,6 +248,11 @@ func (b *Builder) Do(ctx context.Context, root *Action) {
 
 	wg.Wait()
 
+	if tokens != totalTokens || concurrentProcesses != 0 {
+		base.Fatalf("internal error: tokens not restored at end of build: tokens: %d, totalTokens: %d, concurrentProcesses: %d",
+			tokens, totalTokens, concurrentProcesses)
+	}
+
 	// Write action graph again, this time with timing information.
 	writeActionGraph()
 }

src/cmd/go/internal/work/gc.go

@@ -227,6 +227,7 @@ func compilerConcurrency() (int, func()) {
 	return c, func() {
 		tokensMu.Lock()
 		defer tokensMu.Unlock()
+		concurrentProcesses--
 		tokens += c
 	}
 }
@@ -235,6 +236,7 @@ var maxCompilerConcurrency = runtime.GOMAXPROCS(0) // max value we will use for
 
 var (
 	tokensMu            sync.Mutex
+	totalTokens         int // total number of tokens: this is used for checking that we get them all back in the end
 	tokens              int // number of available tokens
 	concurrentProcesses int // number of currently running compiles
 )
@@ -246,6 +248,7 @@ func initCompilerConcurrencyPool() {
 	// than what it was when we capped the concurrency to 4.
 	oldConcurrencyCap := min(4, maxCompilerConcurrency)
 	tokens = oldConcurrencyCap * cfg.BuildP
+	totalTokens = tokens
 }
 
 // trimpath returns the -trimpath argument to use