dev: have doctor advise to set a particular tmpdir

Bazel's default behavior of rooting the `tmpdir` to an "in-sandbox"
directory has been a point of confusion for CRL developers. The sandbox
directory does not exist after the test is run (unless `--sandbox_debug`
is provided), which is sometimes confusing for folks who expect their
test's temp files to be present where the logs suggest they should be
(see #82413). Furthermore, the long `tmpdir` used in these cases breaks
tests that create Unix sockets on OS's where Unix sockets have a maximum
path length.

Avoid these problems by having `doctor` just tell you to manually set a
`test_tmpdir`. We add `/tmp` to `gitignore` in case people want to root
it at the `tmp` directory in their checkout.

Closes #72918.
Closes #82413.

Release note: None

.bazelignore

@@ -3,4 +3,5 @@ bin
 build/builder_home
 lib
 pkg/ui/node_modules
+tmp
 vendor

.gitignore

@@ -14,6 +14,7 @@ artifacts
 /bin.*
 /lib
 /lib.*
+/tmp
 .buildinfo
 # cockroach-data, cockroach{,.race}-{darwin,linux,windows}-*
 /cockroach*

dev

@@ -3,7 +3,7 @@
 set -euo pipefail
 
 # Bump this counter to force rebuilding `dev` on all machines.
-DEV_VERSION=34
+DEV_VERSION=35
 
 THIS_DIR=$(cd "$(dirname "$0")" && pwd)
 BINARY_DIR=$THIS_DIR/bin/dev-versions

pkg/cmd/dev/doctor.go

@@ -32,7 +32,7 @@ const (
 	// doctorStatusVersion is the current "version" of the status checks performed
 	// by `dev doctor``. Increasing it will force doctor to be re-run before other
 	// dev commands can be run.
-	doctorStatusVersion = 4
+	doctorStatusVersion = 5
 
 	noCacheFlag = "no-cache"
 )
@@ -287,6 +287,15 @@ slightly slower and introduce a noticeable delay in first-time build setup.`
 		log.Println(failedNogoTestMsg)
 	}
 
+	// Check whether the user has configured a custom tmpdir.
+	present := d.checkLinePresenceInBazelRcUser(workspace, "test --test_tmpdir=")
+	if !present {
+		failures = append(failures, "You haven't configured a tmpdir for your tests.\n"+
+			"Please add a `test --test_tmpdir=/PATH/TO/TMPDIR` line to your .bazelrc.user:\n"+
+			fmt.Sprintf("    echo \"test --test_tmpdir=%s\" >> .bazelrc.user\n", filepath.Join(workspace, "tmp"))+
+			"(You can choose any directory as a tmpdir.)")
+	}
+
 	// We want to make sure there are no other failures before trying to
 	// set up the cache.
 	if !noCache && len(failures) == 0 {
@@ -359,3 +368,19 @@ func (d *dev) checkPresenceInBazelRc(expectedBazelRcLine string) (string, error)
 	}
 	return errString, nil
 }
+
+// checkLinePresenceInBazelRcUser checks whether the .bazelrc.user file
+// contains a line starting with the given prefix. Returns true iff a matching
+// line is in the file. Failures to find the file are ignored.
+func (d *dev) checkLinePresenceInBazelRcUser(workspace, expectedSubstr string) bool {
+	contents, err := d.os.ReadFile(filepath.Join(workspace, ".bazelrc.user"))
+	if err != nil {
+		return false
+	}
+	for _, line := range strings.Split(contents, "\n") {
+		if strings.HasPrefix(line, expectedSubstr) {
+			return true
+		}
+	}
+	return false
+}