handle when zig path link exists as a non-symlink entry

test.zig

@@ -126,6 +126,41 @@ pub fn main() !u8 {
         dumpExecResult(result);
         try testing.expect(std.mem.eql(u8, result.stdout, "0.7.0\n"));
     }
+
+    // verify we print a nice error message if we can't update the symlink
+    // because it's a directory
+    {
+        const zig_exe_link = comptime "scratch" ++ sep ++ "bin" ++ sep ++ "zig" ++ builtin.target.exeFileExt();
+
+        if (std.fs.cwd().access(zig_exe_link, .{})) {
+            try std.fs.cwd().deleteFile(zig_exe_link);
+        } else |err| switch (err) {
+            error.FileNotFound => {},
+            else => |e| return e,
+        }
+        try std.fs.cwd().makeDir(zig_exe_link);
+
+        const result = try runCaptureOuts(allocator, zigup_args ++ &[_][]const u8{ "default", "0.7.0" });
+        defer {
+            allocator.free(result.stdout);
+            allocator.free(result.stderr);
+        }
+        dumpExecResult(result);
+        switch (result.term) {
+            .Exited => |code| try testing.expectEqual(@as(u8, 1), code),
+            else => |term| std.debug.panic("unexpected exit {}", .{term}),
+        }
+        if (builtin.os.tag == .windows) {
+            try testing.expect(std.mem.containsAtLeast(u8, result.stderr, 1, "unable to create the exe link, the path '"));
+            try testing.expect(std.mem.containsAtLeast(u8, result.stderr, 1, "' is a directory"));
+        } else {
+            try testing.expect(std.mem.containsAtLeast(u8, result.stderr, 1, "unable to update/overwrite the 'zig' PATH symlink, the file '"));
+            try testing.expect(std.mem.containsAtLeast(u8, result.stderr, 1, "' already exists and is not a symlink"));
+        }
+
+        try std.fs.cwd().deleteDir(zig_exe_link);
+    }
+
     {
         const result = try runCaptureOuts(allocator, zigup_args ++ &[_][]const u8{ "fetch", "0.7.0" });
         defer {

zigup.zig

@@ -463,6 +463,13 @@ pub fn loggyUpdateSymlink(target_path: []const u8, sym_link_path: []const u8, fl
         try std.os.unlink(sym_link_path);
     } else |e| switch (e) {
         error.FileNotFound => {},
+        error.NotLink => {
+            std.debug.print(
+                "unable to update/overwrite the 'zig' PATH symlink, the file '{s}' already exists and is not a symlink\n",
+                .{ sym_link_path},
+            );
+            std.os.exit(1);
+        },
         else => return e,
     }
     try loggySymlinkAbsolute(target_path, sym_link_path, flags);
@@ -865,7 +872,16 @@ fn createExeLink(link_target: []const u8, path_link: []const u8) !void {
         std.debug.print("Error: path_link (size {}) is too large (max {})\n", .{ path_link.len, std.fs.MAX_PATH_BYTES });
         return error.AlreadyReported;
     }
-    const file = try std.fs.cwd().createFile(path_link, .{});
+    const file = std.fs.cwd().createFile(path_link, .{}) catch |err| switch (err) {
+        error.IsDir => {
+            std.debug.print(
+                "unable to create the exe link, the path '{s}' is a directory\n",
+                .{ path_link},
+            );
+            std.os.exit(1);
+        },
+        else => |e| return e,
+    };
     defer file.close();
     try file.writer().writeAll(win32exelink.content[0..win32exelink.exe_offset]);
     try file.writer().writeAll(link_target);