cmd/go: clean -cache -n should not delete cache

Uses the `cfg.BuildN` flag to avoid deleting inside the `if cleanCache`
block. Introduces a test in src/cmd/go/testdata/script.

Fixes #39250

Change-Id: I857c441b1d7aa7c68cfd646d6833e6eaca5b18d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/235140
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/clean/clean.go

@@ -137,20 +137,27 @@ func runClean(cmd *base.Command, args []string) {
 				if cfg.BuildN || cfg.BuildX {
 					b.Showcmd("", "rm -r %s", strings.Join(subdirs, " "))
 				}
-				for _, d := range subdirs {
-					// Only print the first error - there may be many.
-					// This also mimics what os.RemoveAll(dir) would do.
-					if err := os.RemoveAll(d); err != nil && !printedErrors {
-						printedErrors = true
-						base.Errorf("go clean -cache: %v", err)
+				if !cfg.BuildN {
+					for _, d := range subdirs {
+						// Only print the first error - there may be many.
+						// This also mimics what os.RemoveAll(dir) would do.
+						if err := os.RemoveAll(d); err != nil && !printedErrors {
+							printedErrors = true
+							base.Errorf("go clean -cache: %v", err)
+						}
 					}
 				}
 			}
 
 			logFile := filepath.Join(dir, "log.txt")
-			if err := os.RemoveAll(logFile); err != nil && !printedErrors {
-				printedErrors = true
-				base.Errorf("go clean -cache: %v", err)
+			if cfg.BuildN || cfg.BuildX {
+				b.Showcmd("", "rm -f %s", logFile)
+			}
+			if !cfg.BuildN {
+				if err := os.RemoveAll(logFile); err != nil && !printedErrors {
+					printedErrors = true
+					base.Errorf("go clean -cache: %v", err)
+				}
 			}
 		}
 	}

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

@@ -0,0 +1,25 @@
+# We're testing cache behavior, so start with a clean GOCACHE.
+env GOCACHE=$WORK/cache
+
+# Build something so that the cache gets populates
+go build main.go
+
+# Check that cache contains directories before running
+exists $GOCACHE/00
+
+# Run go clean -cache -n and ensure that directories weren't deleted
+go clean -cache -n
+exists $GOCACHE/00
+
+# Re-run go clean cache without the -n flag go ensure that directories were properly removed
+go clean -cache
+! exists $GOCACHE/00
+
+-- main.go --
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("hello!")
+}