cmd/go: more refined handling of cmd/compile magic environment variables

Per discussion with David Chase, need to check GOSSAHASH$n
for increasing n until one is missing. Also if GSHS_LOGFILE is set,
the compiler writes to that file, so arrange never to cache in that case.

Change-Id: I3931b4e296251b99abab9bbbbbdcf94ae8c1e2a6
Reviewed-on: https://go-review.googlesource.com/77111
Reviewed-by: David Chase <drchase@google.com>

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

@@ -223,14 +223,30 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 			"GOCLOBBERDEADHASH",
 			"GOSSAFUNC",
 			"GO_SSA_PHI_LOC_CUTOFF",
-			"GSHS_LOGFILE",
 			"GOSSAHASH",
 		}
 		for _, env := range magic {
 			if x := os.Getenv(env); x != "" {
 				fmt.Fprintf(h, "magic %s=%s\n", env, x)
 			}
 		}
+		if os.Getenv("GOSSAHASH") != "" {
+			for i := 0; ; i++ {
+				env := fmt.Sprintf("GOSSAHASH%d", i)
+				x := os.Getenv(env)
+				if x == "" {
+					break
+				}
+				fmt.Fprintf(h, "magic %s=%s\n", env, x)
+			}
+		}
+		if os.Getenv("GSHS_LOGFILE") != "" {
+			// Clumsy hack. Compiler writes to this log file,
+			// so do not allow use of cache at all.
+			// We will still write to the cache but it will be
+			// essentially unfindable.
+			fmt.Fprintf(h, "nocache %d\n", time.Now().UnixNano())
+		}
 	}
 
 	// Input files.