Fix Go apply/vendor on Windows read-only module cache (#346)

crates/socket-patch-core/src/patch/copy_tree.rs

@@ -18,10 +18,12 @@ fn to_io<E: std::fmt::Display>(e: E) -> std::io::Error {
 /// Directories are created fresh (writable, subject to umask) rather than
 /// mirroring the cache's read-only modes, so the copy can be patched and later
 /// removed without a chmod dance. File *contents* are copied via
-/// `std::fs::copy`, which also carries the source's mode bits (often `0o444` in
-/// the cache); the downstream apply pipeline replaces files via stage +
-/// rename (no write grant needed), and
-/// [`remove_tree`] relaxes perms on cleanup. Symlinks / specials are skipped —
+/// `std::fs::copy`, which also carries the source's permissions (`0o444` /
+/// the Windows read-only attribute in Go's module cache), so each copied file
+/// that lands read-only is then granted owner-write: the copy is a private,
+/// freshly created inode, and the apply pipeline's stage + rename cannot
+/// replace a read-only destination on Windows (`MoveFileEx` refuses it with
+/// `ERROR_ACCESS_DENIED`, #346). [`remove_tree`] relaxes perms on cleanup. Symlinks / specials are skipped —
 /// crates.io registry and Go module-cache sources contain none, and copying a
 /// dangling link would be unsafe.
 pub(crate) async fn fresh_copy(
@@ -78,11 +80,35 @@ fn copy_tree_blocking(src: &Path, dst: &Path, skip_file_name: Option<&str>) -> s
                 }
             }
             std::fs::copy(entry.path(), &target)?;
+            make_owner_writable(&target)?;
         }
     }
     Ok(())
 }
 
+/// Grant owner-write on a file [`copy_tree_blocking`] just created, when
+/// the copy carried a read-only mode/attribute over from its source. Only
+/// ever called on the fresh inode `std::fs::copy` wrote, never on a source
+/// or a link, so no shared inode's permissions change.
+fn make_owner_writable(path: &Path) -> std::io::Result<()> {
+    let mut perms = std::fs::metadata(path)?.permissions();
+    if !perms.readonly() {
+        return Ok(());
+    }
+    #[cfg(unix)]
+    {
+        use std::os::unix::fs::PermissionsExt;
+        perms.set_mode(perms.mode() | 0o200);
+    }
+    #[cfg(not(unix))]
+    {
+        // Windows: clears the read-only attribute, nothing else.
+        #[allow(clippy::permissions_set_readonly_false)]
+        perms.set_readonly(false);
+    }
+    std::fs::set_permissions(path, perms)
+}
+
 /// The previous [`copy_tree_blocking`], which created every file's parent,
 /// kept as the equivalence oracle.
 #[cfg(test)]
@@ -113,6 +139,7 @@ fn copy_tree_blocking_reference(
                 std::fs::create_dir_all(p)?;
             }
             std::fs::copy(entry.path(), &target)?;
+            make_owner_writable(&target)?;
         }
     }
     Ok(())
@@ -513,6 +540,37 @@ mod tests {
         fs::set_permissions(src.path().join("ro"), fs::Permissions::from_mode(0o755)).unwrap();
     }
 
+    /// #346: a read-only source file (Go's module cache; the Windows
+    /// read-only attribute) yields an owner-writable copy, so the apply
+    /// pipeline's stage + rename can replace it on every platform. The
+    /// source keeps its read-only permission.
+    #[tokio::test]
+    async fn fresh_copy_files_are_writable_even_from_readonly_source() {
+        let src = tempfile::tempdir().unwrap();
+        let dst = tempfile::tempdir().unwrap();
+        let d = dst.path().join("copy");
+        let f = src.path().join("lib.go");
+        fs::write(&f, b"package lib\n").unwrap();
+        let mut ro = fs::metadata(&f).unwrap().permissions();
+        ro.set_readonly(true);
+        fs::set_permissions(&f, ro).unwrap();
+
+        fresh_copy(src.path(), &d, None).await.unwrap();
+
+        let copied = fs::metadata(d.join("lib.go")).unwrap().permissions();
+        assert!(!copied.readonly(), "the copy must be writable: {copied:?}");
+        #[cfg(unix)]
+        assert_eq!(copied.mode() & 0o777, 0o644, "only owner-write is added");
+        assert!(fs::metadata(&f).unwrap().permissions().readonly());
+        assert_eq!(fs::read(d.join("lib.go")).unwrap(), b"package lib\n");
+
+        // Let the temp dir clean up on Windows.
+        let mut rw = fs::metadata(&f).unwrap().permissions();
+        #[allow(clippy::permissions_set_readonly_false)]
+        rw.set_readonly(false);
+        fs::set_permissions(&f, rw).unwrap();
+    }
+
     #[cfg(unix)]
     #[tokio::test]
     async fn remove_tree_does_not_follow_symlink_out_of_tree() {

crates/socket-patch-core/src/patch/redirect/golang_local.rs

@@ -874,6 +874,52 @@ mod tests {
         assert_eq!(e.version.as_deref(), Some(VERSION));
     }
 
+    /// #346: Go extracts module-cache files read-only (on Windows, the
+    /// read-only attribute). The copy must still be patchable — Windows'
+    /// rename refuses to replace a read-only destination — and the cache
+    /// itself must stay read-only and pristine.
+    #[tokio::test]
+    async fn test_apply_redirect_over_a_read_only_module_cache() {
+        let (dir, blobs, pristine, files, _after) = fixture().await;
+        let root = dir.path();
+        let set_readonly = |readonly: bool| {
+            for name in ["bar.go", "go.mod"] {
+                let path = pristine.join(name);
+                let mut perms = std::fs::metadata(&path).unwrap().permissions();
+                #[allow(clippy::permissions_set_readonly_false)]
+                perms.set_readonly(readonly);
+                std::fs::set_permissions(&path, perms).unwrap();
+            }
+        };
+        set_readonly(true);
+        let sources = PatchSources::blobs_only(&blobs);
+
+        for base in [GO_PATCHES_DIR, ".socket/vendor/golang/u"] {
+            let result = apply_go_redirect(
+                PURL,
+                MODULE,
+                VERSION,
+                &pristine,
+                root,
+                base,
+                &files,
+                &sources,
+                false,
+                MismatchPolicy::Warn,
+            )
+            .await;
+            assert!(result.success, "{base}: apply failed: {:?}", result.error);
+            let copy = root.join(base).join("github.com/foo/bar@v1.4.2");
+            assert_eq!(std::fs::read(copy.join("bar.go")).unwrap(), PATCHED);
+        }
+        assert_eq!(std::fs::read(pristine.join("bar.go")).unwrap(), PRISTINE);
+        assert!(std::fs::metadata(pristine.join("bar.go"))
+            .unwrap()
+            .permissions()
+            .readonly());
+        set_readonly(false);
+    }
+
     #[tokio::test]
     async fn test_apply_is_idempotent_byte_for_byte() {
         let (dir, blobs, pristine, files, _after) = fixture().await;