internal/synctest: new package for testing concurrent code

Add an internal (for now) implementation of testing/synctest.

The synctest.Run function executes a tree of goroutines in an
isolated environment using a fake clock. The synctest.Wait function
allows a test to wait for all other goroutines within the test
to reach a blocking point.

For #67434
For #69687

Change-Id: Icb39e54c54cece96517e58ef9cfb18bf68506cfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/591997
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 18 insertions, 3 deletions

diff --git a/src/runtime/select.go b/src/runtime/select.go
index 2e86c85493..0b1d144951 100644
--- a/src/runtime/select.go
+++ b/src/runtime/select.go
@@ -120,6 +120,7 @@ func block() {
 // Also, if the chosen scase was a receive operation, it reports whether
 // a value was received.
 func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool) {
+	gp := getg()
 	if debugSelect {
 		print("select: cas0=", cas0, "\n")
 	}
@@ -165,6 +166,7 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, blo
 
 	// generate permuted order
 	norder := 0
+	allSynctest := true
 	for i := range scases {
 		cas := &scases[i]
 
@@ -174,6 +176,14 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, blo
 			continue
 		}
 
+		if cas.c.synctest {
+			if getg().syncGroup == nil {
+				panic(plainError("select on synctest channel from outside bubble"))
+			}
+		} else {
+			allSynctest = false
+		}
+
 		if cas.c.timer != nil {
 			cas.c.timer.maybeRunChan()
 		}
@@ -186,6 +196,13 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, blo
 	pollorder = pollorder[:norder]
 	lockorder = lockorder[:norder]
 
+	waitReason := waitReasonSelect
+	if gp.syncGroup != nil && allSynctest {
+		// Every channel selected on is in a synctest bubble,
+		// so this goroutine will count as idle while selecting.
+		waitReason = waitReasonSynctestSelect
+	}
+
 	// sort the cases by Hchan address to get the locking order.
 	// simple heap sort, to guarantee n log n time and constant stack footprint.
 	for i := range lockorder {
@@ -235,7 +252,6 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, blo
 	sellock(scases, lockorder)
 
 	var (
-		gp     *g
 		sg     *sudog
 		c      *hchan
 		k      *scase
@@ -291,7 +307,6 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, blo
 	}
 
 	// pass 2 - enqueue on all chans
-	gp = getg()
 	if gp.waiting != nil {
 		throw("gp.waiting != nil")
 	}
@@ -333,7 +348,7 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, blo
 	// changes and when we set gp.activeStackChans is not safe for
 	// stack shrinking.
 	gp.parkingOnChan.Store(true)
-	gopark(selparkcommit, nil, waitReasonSelect, traceBlockSelect, 1)
+	gopark(selparkcommit, nil, waitReason, traceBlockSelect, 1)
 	gp.activeStackChans = false
 
 	sellock(scases, lockorder)