cmd/api: reduce parallel 'go list' invocations to a constant

'go list' has its own internal parallelism, so invoking in in parallel
can produce up to quadratic peak memory usage.

Running 'go list' is also very I/O-intensive, so the higher
parallelism does substantially improve latency; unfortunately, we lack
a good way to balance latency against memory footprint, so we need to
sacrifice some latency for reliability.

Fixes #49957.

Change-Id: Ib53990b46acf4cc67a9141644d97282964d6442d
Reviewed-on: https://go-review.googlesource.com/c/go/+/380994
Trust: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: Bryan C. Mills

src/cmd/api/goapi.go

@@ -459,8 +459,11 @@ type listImports struct {
 
 var listCache sync.Map // map[string]listImports, keyed by contextName
 
-// listSem is a semaphore restricting concurrent invocations of 'go list'.
-var listSem = make(chan semToken, ((runtime.GOMAXPROCS(0)-1)/2)+1)
+// listSem is a semaphore restricting concurrent invocations of 'go list'. 'go
+// list' has its own internal concurrency, so we use a hard-coded constant (to
+// allow the I/O-intensive phases of 'go list' to overlap) instead of scaling
+// all the way up to GOMAXPROCS.
+var listSem = make(chan semToken, 2)
 
 type semToken struct{}