std.Build.Watch: introduce special file "." to watch entire dir

And use it to implement InstallDir Step watch integration.

I'm not seeing any events triggered when I run `mkdir` in the watched
directory, however, and I have not yet figured out why.

3 files changed, 75 insertions, 21 deletions

diff --git a/lib/std/Build/Step.zig b/lib/std/Build/Step.zig
index e41912d548..a2640492ba 100644
--- a/lib/std/Build/Step.zig
+++ b/lib/std/Build/Step.zig
@@ -160,6 +160,7 @@ pub const Inputs = struct {
     };
 
     pub const Table = std.ArrayHashMapUnmanaged(Build.Cache.Path, Files, Build.Cache.Path.TableAdapter, false);
+    /// The special file name "." means any changes inside the directory.
     pub const Files = std.ArrayListUnmanaged([]const u8);
 
     pub fn populated(inputs: *Inputs) bool {
@@ -611,8 +612,9 @@ pub fn clearWatchInputs(step: *Step) void {
     step.inputs.clear(gpa);
 }
 
-pub fn addWatchInput(step: *Step, lazy_path: Build.LazyPath) Allocator.Error!void {
-    switch (lazy_path) {
+/// Places a *file* dependency on the path.
+pub fn addWatchInput(step: *Step, lazy_file: Build.LazyPath) Allocator.Error!void {
+    switch (lazy_file) {
         .src_path => |src_path| try addWatchInputFromBuilder(step, src_path.owner, src_path.sub_path),
         .dependency => |d| try addWatchInputFromBuilder(step, d.dependency.builder, d.sub_path),
         .cwd_relative => |path_string| {
@@ -629,6 +631,34 @@ pub fn addWatchInput(step: *Step, lazy_path: Build.LazyPath) Allocator.Error!voi
     }
 }
 
+/// Any changes inside the directory will trigger invalidation.
+///
+/// See also `addDirectoryWatchInputFromPath` which takes a `Build.Cache.Path` instead.
+pub fn addDirectoryWatchInput(step: *Step, lazy_directory: Build.LazyPath) Allocator.Error!void {
+    switch (lazy_directory) {
+        .src_path => |src_path| try addDirectoryWatchInputFromBuilder(step, src_path.owner, src_path.sub_path),
+        .dependency => |d| try addDirectoryWatchInputFromBuilder(step, d.dependency.builder, d.sub_path),
+        .cwd_relative => |path_string| {
+            try addDirectoryWatchInputFromPath(step, .{
+                .root_dir = .{
+                    .path = null,
+                    .handle = std.fs.cwd(),
+                },
+                .sub_path = path_string,
+            });
+        },
+        // Nothing to watch because this dependency edge is modeled instead via `dependants`.
+        .generated => {},
+    }
+}
+
+/// Any changes inside the directory will trigger invalidation.
+///
+/// See also `addDirectoryWatchInput` which takes a `Build.LazyPath` instead.
+pub fn addDirectoryWatchInputFromPath(step: *Step, path: Build.Cache.Path) !void {
+    return addWatchInputFromPath(step, path, ".");
+}
+
 fn addWatchInputFromBuilder(step: *Step, builder: *Build, sub_path: []const u8) !void {
     return addWatchInputFromPath(step, .{
         .root_dir = builder.build_root,
@@ -636,6 +666,13 @@ fn addWatchInputFromBuilder(step: *Step, builder: *Build, sub_path: []const u8)
     }, std.fs.path.basename(sub_path));
 }
 
+fn addDirectoryWatchInputFromBuilder(step: *Step, builder: *Build, sub_path: []const u8) !void {
+    return addDirectoryWatchInputFromPath(step, .{
+        .root_dir = builder.build_root,
+        .sub_path = sub_path,
+    });
+}
+
 fn addWatchInputFromPath(step: *Step, path: Build.Cache.Path, basename: []const u8) !void {
     const gpa = step.owner.allocator;
     const gop = try step.inputs.table.getOrPut(gpa, path);
diff --git a/lib/std/Build/Step/InstallDir.zig b/lib/std/Build/Step/InstallDir.zig
index ca38e09ec1..9cd71e7828 100644
--- a/lib/std/Build/Step/InstallDir.zig
+++ b/lib/std/Build/Step/InstallDir.zig
@@ -59,12 +59,14 @@ fn make(step: *Step, prog_node: std.Progress.Node) !void {
     _ = prog_node;
     const b = step.owner;
     const install_dir: *InstallDir = @fieldParentPtr("step", step);
+    step.clearWatchInputs();
     const arena = b.allocator;
     const dest_prefix = b.getInstallPath(install_dir.options.install_dir, install_dir.options.install_subdir);
-    const src_dir_path = install_dir.options.source_dir.getPath2(b, step);
-    var src_dir = b.build_root.handle.openDir(src_dir_path, .{ .iterate = true }) catch |err| {
-        return step.fail("unable to open source directory '{}{s}': {s}", .{
-            b.build_root, src_dir_path, @errorName(err),
+    const src_dir_path = install_dir.options.source_dir.getPath3(b, step);
+    try step.addDirectoryWatchInput(install_dir.options.source_dir);
+    var src_dir = src_dir_path.root_dir.handle.openDir(src_dir_path.subPathOpt() orelse ".", .{ .iterate = true }) catch |err| {
+        return step.fail("unable to open source directory '{}': {s}", .{
+            src_dir_path, @errorName(err),
         });
     };
     defer src_dir.close();
@@ -88,12 +90,16 @@ fn make(step: *Step, prog_node: std.Progress.Node) !void {
         }
 
         // relative to src build root
-        const src_sub_path = b.pathJoin(&.{ src_dir_path, entry.path });
+        const src_sub_path = try src_dir_path.join(arena, entry.path);
         const dest_path = b.pathJoin(&.{ dest_prefix, entry.path });
         const cwd = fs.cwd();
 
         switch (entry.kind) {
-            .directory => try cwd.makePath(dest_path),
+            .directory => {
+                const subdir_path = try src_dir_path.join(arena, entry.path);
+                try step.addDirectoryWatchInputFromPath(subdir_path);
+                try cwd.makePath(dest_path);
+            },
             .file => {
                 for (install_dir.options.blank_extensions) |ext| {
                     if (mem.endsWith(u8, entry.path, ext)) {
@@ -103,14 +109,14 @@ fn make(step: *Step, prog_node: std.Progress.Node) !void {
                 }
 
                 const prev_status = fs.Dir.updateFile(
-                    b.build_root.handle,
-                    src_sub_path,
+                    src_sub_path.root_dir.handle,
+                    src_sub_path.sub_path,
                     cwd,
                     dest_path,
                     .{},
                 ) catch |err| {
-                    return step.fail("unable to update file from '{}{s}' to '{s}': {s}", .{
-                        b.build_root, src_sub_path, dest_path, @errorName(err),
+                    return step.fail("unable to update file from '{}' to '{s}': {s}", .{
+                        src_sub_path, dest_path, @errorName(err),
                     });
                 };
                 all_cached = all_cached and prev_status == .fresh;
diff --git a/lib/std/Build/Watch.zig b/lib/std/Build/Watch.zig
index 2ffcd27ca7..e9b4449748 100644
--- a/lib/std/Build/Watch.zig
+++ b/lib/std/Build/Watch.zig
@@ -12,10 +12,13 @@ generation: Generation,
 
 pub const fan_mask: std.os.linux.fanotify.MarkMask = .{
     .CLOSE_WRITE = true,
+    .CREATE = true,
     .DELETE = true,
+    .DELETE_SELF = true,
+    .EVENT_ON_CHILD = true,
     .MOVED_FROM = true,
     .MOVED_TO = true,
-    .EVENT_ON_CHILD = true,
+    .MOVE_SELF = true,
 };
 
 pub const init: Watch = .{
@@ -32,6 +35,7 @@ pub const init: Watch = .{
 const DirTable = std.ArrayHashMapUnmanaged(Cache.Path, void, Cache.Path.TableAdapter, false);
 
 const HandleTable = std.ArrayHashMapUnmanaged(LinuxFileHandle, ReactionSet, LinuxFileHandle.Adapter, false);
+/// Special key of "." means any changes in this directory trigger the steps.
 const ReactionSet = std.StringArrayHashMapUnmanaged(StepSet);
 const StepSet = std.AutoArrayHashMapUnmanaged(*Step, Generation);
 
@@ -149,14 +153,10 @@ pub fn markDirtySteps(w: *Watch, gpa: Allocator) !bool {
                     const file_name = std.mem.span(file_name_z);
                     const lfh: Watch.LinuxFileHandle = .{ .handle = file_handle };
                     if (w.handle_table.getPtr(lfh)) |reaction_set| {
-                        if (reaction_set.getPtr(file_name)) |step_set| {
-                            for (step_set.keys()) |step| {
-                                if (step.state != .precheck_done) {
-                                    step.recursiveReset(gpa);
-                                    any_dirty = true;
-                                }
-                            }
-                        }
+                        if (reaction_set.getPtr(".")) |glob_set|
+                            any_dirty = markStepSetDirty(gpa, glob_set, any_dirty);
+                        if (reaction_set.getPtr(file_name)) |step_set|
+                            any_dirty = markStepSetDirty(gpa, step_set, any_dirty);
                     }
                 },
                 else => |t| std.log.warn("unexpected fanotify event '{s}'", .{@tagName(t)}),
@@ -187,3 +187,14 @@ fn markAllFilesDirty(w: *Watch, gpa: Allocator) void {
         }
     }
 }
+
+fn markStepSetDirty(gpa: Allocator, step_set: *StepSet, any_dirty: bool) bool {
+    var this_any_dirty = false;
+    for (step_set.keys()) |step| {
+        if (step.state != .precheck_done) {
+            step.recursiveReset(gpa);
+            this_any_dirty = true;
+        }
+    }
+    return any_dirty or this_any_dirty;
+}