Commit 640a130651

Kevin Boulain <kevin@boula.in>
2025-06-07 18:58:24
std.Build.Watch: key fanotify file descriptors by mount id
Since marks are limited to a single filesystem. Context: https://github.com/ziglang/zig/issues/20670 Original pull request: https://github.com/ziglang/zig/pull/20672 Co-authored-by: Maciej 'vesim' Kuliński <vesim809@pm.me>
1 parent 1df79ab
Changed files (1)
lib
std
Build
lib/std/Build/Watch.zig
@@ -33,9 +33,12 @@ const Os = switch (builtin.os.tag) {
 
         /// Keyed differently but indexes correspond 1:1 with `dir_table`.
         handle_table: HandleTable,
-        poll_fds: [1]posix.pollfd,
+        /// fanotify file descriptors are keyed by mount id since marks
+        /// are limited to a single filesystem.
+        poll_fds: std.AutoArrayHashMapUnmanaged(MountId, posix.pollfd),
 
-        const HandleTable = std.ArrayHashMapUnmanaged(FileHandle, ReactionSet, FileHandle.Adapter, false);
+        const MountId = i32;
+        const HandleTable = std.ArrayHashMapUnmanaged(FileHandle, struct { mount_id: MountId, reaction_set: ReactionSet }, FileHandle.Adapter, false);
 
         const fan_mask: std.os.linux.fanotify.MarkMask = .{
             .CLOSE_WRITE = true,
@@ -92,30 +95,12 @@ const Os = switch (builtin.os.tag) {
         };
 
         fn init() !Watch {
-            const fan_fd = std.posix.fanotify_init(.{
-                .CLASS = .NOTIF,
-                .CLOEXEC = true,
-                .NONBLOCK = true,
-                .REPORT_NAME = true,
-                .REPORT_DIR_FID = true,
-                .REPORT_FID = true,
-                .REPORT_TARGET_FID = true,
-            }, 0) catch |err| switch (err) {
-                error.UnsupportedFlags => fatal("fanotify_init failed due to old kernel; requires 5.17+", .{}),
-                else => |e| return e,
-            };
             return .{
                 .dir_table = .{},
                 .os = switch (builtin.os.tag) {
                     .linux => .{
                         .handle_table = .{},
-                        .poll_fds = .{
-                            .{
-                                .fd = fan_fd,
-                                .events = std.posix.POLL.IN,
-                                .revents = undefined,
-                            },
-                        },
+                        .poll_fds = .{},
                     },
                     else => {},
                 },
@@ -123,22 +108,20 @@ const Os = switch (builtin.os.tag) {
             };
         }
 
-        fn getDirHandle(gpa: Allocator, path: std.Build.Cache.Path) !FileHandle {
+        fn getDirHandle(gpa: Allocator, path: std.Build.Cache.Path, mount_id: *MountId) !FileHandle {
             var file_handle_buffer: [@sizeOf(std.os.linux.file_handle) + 128]u8 align(@alignOf(std.os.linux.file_handle)) = undefined;
-            var mount_id: i32 = undefined;
             var buf: [std.fs.max_path_bytes]u8 = undefined;
             const adjusted_path = if (path.sub_path.len == 0) "./" else std.fmt.bufPrint(&buf, "{s}/", .{
                 path.sub_path,
             }) catch return error.NameTooLong;
             const stack_ptr: *std.os.linux.file_handle = @ptrCast(&file_handle_buffer);
             stack_ptr.handle_bytes = file_handle_buffer.len - @sizeOf(std.os.linux.file_handle);
-            try posix.name_to_handle_at(path.root_dir.handle.fd, adjusted_path, stack_ptr, &mount_id, std.os.linux.AT.HANDLE_FID);
+            try posix.name_to_handle_at(path.root_dir.handle.fd, adjusted_path, stack_ptr, mount_id, std.os.linux.AT.HANDLE_FID);
             const stack_lfh: FileHandle = .{ .handle = stack_ptr };
             return stack_lfh.clone(gpa);
         }
 
-        fn markDirtySteps(w: *Watch, gpa: Allocator) !bool {
-            const fan_fd = w.os.getFanFd();
+        fn markDirtySteps(w: *Watch, gpa: Allocator, fan_fd: posix.fd_t) !bool {
             const fanotify = std.os.linux.fanotify;
             const M = fanotify.event_metadata;
             var events_buf: [256 + 4096]u8 = undefined;
@@ -167,10 +150,10 @@ const Os = switch (builtin.os.tag) {
                             const file_name_z: [*:0]u8 = @ptrCast((&file_handle.f_handle).ptr + file_handle.handle_bytes);
                             const file_name = std.mem.span(file_name_z);
                             const lfh: FileHandle = .{ .handle = file_handle };
-                            if (w.os.handle_table.getPtr(lfh)) |reaction_set| {
-                                if (reaction_set.getPtr(".")) |glob_set|
+                            if (w.os.handle_table.getPtr(lfh)) |value| {
+                                if (value.reaction_set.getPtr(".")) |glob_set|
                                     any_dirty = markStepSetDirty(gpa, glob_set, any_dirty);
-                                if (reaction_set.getPtr(file_name)) |step_set|
+                                if (value.reaction_set.getPtr(file_name)) |step_set|
                                     any_dirty = markStepSetDirty(gpa, step_set, any_dirty);
                             }
                         },
@@ -180,19 +163,38 @@ const Os = switch (builtin.os.tag) {
             }
         }
 
-        fn getFanFd(os: *const @This()) posix.fd_t {
-            return os.poll_fds[0].fd;
-        }
-
         fn update(w: *Watch, gpa: Allocator, steps: []const *Step) !void {
-            const fan_fd = w.os.getFanFd();
             // Add missing marks and note persisted ones.
             for (steps) |step| {
                 for (step.inputs.table.keys(), step.inputs.table.values()) |path, *files| {
                     const reaction_set = rs: {
                         const gop = try w.dir_table.getOrPut(gpa, path);
                         if (!gop.found_existing) {
-                            const dir_handle = try Os.getDirHandle(gpa, path);
+                            var mount_id: MountId = undefined;
+                            const dir_handle = try Os.getDirHandle(gpa, path, &mount_id);
+                            const fan_fd = blk: {
+                                const fd_gop = try w.os.poll_fds.getOrPut(gpa, mount_id);
+                                if (!fd_gop.found_existing) {
+                                    const fan_fd = std.posix.fanotify_init(.{
+                                        .CLASS = .NOTIF,
+                                        .CLOEXEC = true,
+                                        .NONBLOCK = true,
+                                        .REPORT_NAME = true,
+                                        .REPORT_DIR_FID = true,
+                                        .REPORT_FID = true,
+                                        .REPORT_TARGET_FID = true,
+                                    }, 0) catch |err| switch (err) {
+                                        error.UnsupportedFlags => fatal("fanotify_init failed due to old kernel; requires 5.17+", .{}),
+                                        else => |e| return e,
+                                    };
+                                    fd_gop.value_ptr.* = .{
+                                        .fd = fan_fd,
+                                        .events = std.posix.POLL.IN,
+                                        .revents = undefined,
+                                    };
+                                }
+                                break :blk fd_gop.value_ptr.*.fd;
+                            };
                             // `dir_handle` may already be present in the table in
                             // the case that we have multiple Cache.Path instances
                             // that compare inequal but ultimately point to the same
@@ -204,7 +206,7 @@ const Os = switch (builtin.os.tag) {
                                 _ = w.dir_table.pop();
                             } else {
                                 assert(dh_gop.index == gop.index);
-                                dh_gop.value_ptr.* = .{};
+                                dh_gop.value_ptr.* = .{ .mount_id = mount_id, .reaction_set = .{} };
                                 posix.fanotify_mark(fan_fd, .{
                                     .ADD = true,
                                     .ONLYDIR = true,
@@ -212,9 +214,9 @@ const Os = switch (builtin.os.tag) {
                                     fatal("unable to watch {}: {s}", .{ path, @errorName(err) });
                                 };
                             }
-                            break :rs dh_gop.value_ptr;
+                            break :rs &dh_gop.value_ptr.reaction_set;
                         }
-                        break :rs &w.os.handle_table.values()[gop.index];
+                        break :rs &w.os.handle_table.values()[gop.index].reaction_set;
                     };
                     for (files.items) |basename| {
                         const gop = try reaction_set.getOrPut(gpa, basename);
@@ -229,7 +231,7 @@ const Os = switch (builtin.os.tag) {
                 var i: usize = 0;
                 while (i < w.os.handle_table.entries.len) {
                     {
-                        const reaction_set = &w.os.handle_table.values()[i];
+                        const reaction_set = &w.os.handle_table.values()[i].reaction_set;
                         var step_set_i: usize = 0;
                         while (step_set_i < reaction_set.entries.len) {
                             const step_set = &reaction_set.values()[step_set_i];
@@ -256,6 +258,8 @@ const Os = switch (builtin.os.tag) {
 
                     const path = w.dir_table.keys()[i];
 
+                    const mount_id = w.os.handle_table.values()[i].mount_id;
+                    const fan_fd = w.os.poll_fds.getEntry(mount_id).?.value_ptr.fd;
                     posix.fanotify_mark(fan_fd, .{
                         .REMOVE = true,
                         .ONLYDIR = true,
@@ -272,13 +276,14 @@ const Os = switch (builtin.os.tag) {
         }
 
         fn wait(w: *Watch, gpa: Allocator, timeout: Timeout) !WaitResult {
-            const events_len = try std.posix.poll(&w.os.poll_fds, timeout.to_i32_ms());
-            return if (events_len == 0)
-                .timeout
-            else if (try Os.markDirtySteps(w, gpa))
-                .dirty
-            else
-                .clean;
+            const events_len = try std.posix.poll(w.os.poll_fds.values(), timeout.to_i32_ms());
+            if (events_len == 0)
+                return .timeout;
+            for (w.os.poll_fds.values()) |poll_fd| {
+                if (poll_fd.revents & std.posix.POLL.IN == std.posix.POLL.IN and try Os.markDirtySteps(w, gpa, poll_fd.fd))
+                    return .dirty;
+            }
+            return .clean;
         }
     },
     .windows => struct {
@@ -838,7 +843,11 @@ pub const Match = struct {
 };
 
 fn markAllFilesDirty(w: *Watch, gpa: Allocator) void {
-    for (w.os.handle_table.values()) |reaction_set| {
+    for (w.os.handle_table.values()) |value| {
+        const reaction_set = switch (builtin.os.tag) {
+            .linux => value.reaction_set,
+            else => value,
+        };
         for (reaction_set.values()) |step_set| {
             for (step_set.keys()) |step| {
                 step.recursiveReset(gpa);