cmd/go: go test flag -failfast should be cacheable

src/cmd/go/internal/test/test.go

@@ -119,16 +119,16 @@ elapsed time in the summary line.
 The rule for a match in the cache is that the run involves the same
 test binary and the flags on the command line come entirely from a
 restricted set of 'cacheable' test flags, defined as -benchtime, -cpu,
--list, -parallel, -run, -short, and -v. If a run of go test has any test
-or non-test flags outside this set, the result is not cached. To
-disable test caching, use any test flag or argument other than the
-cacheable flags. The idiomatic way to disable test caching explicitly
-is to use -count=1. Tests that open files within the package's source
-root (usually $GOPATH) or that consult environment variables only
-match future runs in which the files and environment variables are unchanged.
-A cached test result is treated as executing in no time at all,
-so a successful package test result will be cached and reused
-regardless of -timeout setting.
+-list, -parallel, -run, -short, -timeout, -failfast, and -v.
+If a run of go test has any test or non-test flags outside this set,
+the result is not cached. To disable test caching, use any test flag
+or argument other than the cacheable flags. The idiomatic way to disable
+test caching explicitly is to use -count=1. Tests that open files within
+the package's source root (usually $GOPATH) or that consult environment
+variables only match future runs in which the files and environment
+variables are unchanged. A cached test result is treated as executing
+in no time at all,so a successful package test result will be cached and
+reused regardless of -timeout setting.
 
 In addition to the build flags, the flags handled by 'go test' itself are:
 
@@ -1347,6 +1347,7 @@ func (c *runCache) tryCacheWithID(b *work.Builder, a *work.Action, id string) bo
 			"-test.run",
 			"-test.short",
 			"-test.timeout",
+			"-test.failfast",
 			"-test.v":
 			// These are cacheable.
 			// Note that this list is documented above,

src/cmd/go/testdata/script/test_cache_inputs.txt

@@ -108,6 +108,12 @@ go test testcache -run=Benchtime -bench=Benchtime -benchtime=1x
 go test testcache -run=Benchtime -bench=Benchtime -benchtime=1x
 ! stdout '\(cached\)'
 
+# golang.org/issue/47355: that includes the `-failfast` argument.
+go test testcache -run=TestOSArgs -failfast
+! stdout '\(cached\)'
+go test testcache -run=TestOSArgs -failfast
+stdout '\(cached\)'
+
 # Executables within GOROOT and GOPATH should affect caching,
 # even if the test does not stat them explicitly.