Fix bug in reuse_sandbox_directories

#19935 gives a clear repro. The problem appears when a stashed sandbox
contains a directory whose same path is a regular file in a later
execution. If we try to reuse the stashed sandbox we trigger an error if
the old directory contained any files.

This was due to simply calling delete() without checking first if it
was a directory. The fix is to call deleteTree() instead in those cases.
directory.

Fixes #19935

RELNOTES:none
PiperOrigin-RevId: 576889004
Change-Id: I73b145cd574b83c473ffaccd90b675eb5f086c0e

src/main/java/com/google/devtools/build/lib/sandbox/SandboxHelpers.java

@@ -184,6 +184,8 @@ private static void cleanRecursively(
         if (SYMLINK.equals(dirent.getType())
             && absPath.readSymbolicLink().equals(destination.get())) {
           inputsToCreate.remove(pathRelativeToWorkDir);
+        } else if (absPath.isDirectory()) {
+          absPath.deleteTree();
         } else {
           absPath.delete();
         }

src/test/java/com/google/devtools/build/lib/sandbox/SandboxHelpersTest.java

@@ -293,7 +293,19 @@ public void cleanExisting_updatesDirs() throws IOException, InterruptedException
     // outputDir only exists partially
     execRootPath.getRelative(outputDir).getParentDirectory().createDirectoryAndParents();
     execRootPath.getRelative("justSomeDir/thatIsDoomed").createDirectoryAndParents();
-    SandboxHelpers.cleanExisting(rootDir, inputs, inputsToCreate, dirsToCreate, execRootPath);
+    // `thiswillbeafile/output` simulates a directory that was in the stashed dir but whose same
+    // path is used later for a regular file.
+    scratch.dir("/execRoot/thiswillbeafile/output");
+    scratch.file("/execRoot/thiswillbeafile/output/file1");
+    dirsToCreate.add(PathFragment.create("thiswillbeafile"));
+    PathFragment input4 = PathFragment.create("thiswillbeafile/output");
+    SandboxInputs inputs2 =
+        new SandboxInputs(
+            ImmutableMap.of(input1, inputTxt, input2, inputTxt, input3, inputTxt, input4, inputTxt),
+            ImmutableMap.of(),
+            ImmutableMap.of(),
+            ImmutableMap.of());
+    SandboxHelpers.cleanExisting(rootDir, inputs2, inputsToCreate, dirsToCreate, execRootPath);
     assertThat(dirsToCreate).containsExactly(inputDir2, inputDir3, outputDir);
     assertThat(execRootPath.getRelative("existing/directory/with").exists()).isTrue();
     assertThat(execRootPath.getRelative("partial").exists()).isTrue();