package manager: set executable bit

Based on file content. Detects elf magic header or shebang line.

Executable bit is ignored in hash calculation, as it was before this. So
packages hashes are not changed.

Reference:
https://github.com/ziglang/zig/issues/17463#issuecomment-1984798880

Fixes: 17463

Test is here:
7c4600d7bb/src/main.zig (L307)
(if #19500 got accepted I'll move this test to the Fetch.zig)

src/Package/Fetch.zig

@@ -1181,7 +1181,6 @@ fn unpackTarball(f: *Fetch, out_dir: fs.Dir, reader: anytype) RunError!?[]const
     std.tar.pipeToFileSystem(out_dir, reader, .{
         .diagnostics = &diagnostics,
         .strip_components = 0,
-        // https://github.com/ziglang/zig/issues/17463
         .mode_mode = .ignore,
         .exclude_empty_directories = true,
     }) catch |err| return f.fail(f.location_tok, try eb.printString(
@@ -1569,17 +1568,22 @@ fn hashFileFallible(dir: fs.Dir, hashed_file: *HashedFile) HashedFile.Error!void
     var buf: [8000]u8 = undefined;
     var hasher = Manifest.Hash.init(.{});
     hasher.update(hashed_file.normalized_path);
+
     switch (hashed_file.kind) {
         .file => {
             var file = try dir.openFile(hashed_file.fs_path, .{});
             defer file.close();
-            // When implementing https://github.com/ziglang/zig/issues/17463
-            // this will change to hard-coded `false`.
-            hasher.update(&.{ 0, @intFromBool(try isExecutable(file)) });
+            // Hard-coded false executable bit: https://github.com/ziglang/zig/issues/17463
+            hasher.update(&.{ 0, 0 });
+            var file_header: FileHeader = .{};
             while (true) {
                 const bytes_read = try file.read(&buf);
                 if (bytes_read == 0) break;
                 hasher.update(buf[0..bytes_read]);
+                file_header.update(buf[0..bytes_read]);
+            }
+            if (file_header.isExecutable()) {
+                try setExecutable(file);
             }
         },
         .link => {
@@ -1600,19 +1604,12 @@ fn deleteFileFallible(dir: fs.Dir, deleted_file: *DeletedFile) DeletedFile.Error
     try dir.deleteFile(deleted_file.fs_path);
 }
 
-fn isExecutable(file: fs.File) !bool {
-    // When implementing https://github.com/ziglang/zig/issues/17463
-    // this function will not check the mode but instead check if the file is an ELF
-    // file or has a shebang line.
-    if (native_os == .windows) {
-        // Until this is implemented, this could be a false negative on
-        // Windows, which is why we do not yet set executable_bit_only above
-        // when unpacking the tarball.
-        return false;
-    } else {
-        const stat = try file.stat();
-        return (stat.mode & std.posix.S.IXUSR) != 0;
-    }
+fn setExecutable(file: fs.File) !void {
+    if (!std.fs.has_executable_bit) return;
+
+    const S = std.posix.S;
+    const mode = fs.File.default_mode | S.IXUSR | S.IXGRP | S.IXOTH;
+    try file.chmod(mode);
 }
 
 const DeletedFile = struct {
@@ -1635,6 +1632,7 @@ const HashedFile = struct {
         fs.File.OpenError ||
         fs.File.ReadError ||
         fs.File.StatError ||
+        fs.File.ChmodError ||
         fs.Dir.ReadLinkError;
 
     const Kind = enum { file, link };
@@ -1746,3 +1744,37 @@ test {
     _ = Filter;
     _ = FileType;
 }
+
+// Detects executable header: ELF magic header or shebang line.
+const FileHeader = struct {
+    const elf_magic = std.elf.MAGIC;
+    const shebang = "#!";
+
+    header: [@max(elf_magic.len, shebang.len)]u8 = undefined,
+    bytes_read: usize = 0,
+
+    pub fn update(self: *FileHeader, buf: []const u8) void {
+        if (self.bytes_read >= self.header.len) return;
+        const n = @min(self.header.len - self.bytes_read, buf.len);
+        @memcpy(self.header[self.bytes_read..][0..n], buf[0..n]);
+        self.bytes_read += n;
+    }
+
+    pub fn isExecutable(self: *FileHeader) bool {
+        return std.mem.eql(u8, self.header[0..shebang.len], shebang) or
+            std.mem.eql(u8, self.header[0..elf_magic.len], elf_magic);
+    }
+};
+
+test FileHeader {
+    var h: FileHeader = .{};
+    try std.testing.expect(!h.isExecutable());
+
+    h.update(FileHeader.elf_magic[0..2]);
+    try std.testing.expect(!h.isExecutable());
+    h.update(FileHeader.elf_magic[2..4]);
+    try std.testing.expect(h.isExecutable());
+
+    h.update(FileHeader.elf_magic[2..4]);
+    try std.testing.expect(h.isExecutable());
+}