Build puller: wake on a settings file write, retry a failed config follow at the next event, say a capacity of 0 and why

go/buildstep/watch_settings_linux.go

@@ -0,0 +1,58 @@
+//go:build linux
+
+package buildstep
+
+import (
+	"context"
+	"encoding/binary"
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+// watchSettings wakes the puller each time its settings file is written or replaced (inotify on the file's directory, so
+// a rename over it counts): the event the node-engine's refill of the file is, which a parked puller at capacity 0
+// otherwise never sees. No timer: it blocks on the kernel's event. A watch that cannot start is said once.
+func (p *Puller) watchSettings(ctx context.Context) {
+	if p.SettingsFile == "" {
+		return
+	}
+	fd, err := syscall.InotifyInit1(syscall.IN_CLOEXEC | syscall.IN_NONBLOCK)
+	if err != nil {
+		p.logf("the settings file %s cannot be watched, so a refill of it wakes the puller only at another event: %v", p.SettingsFile, err)
+		return
+	}
+	f := os.NewFile(uintptr(fd), "inotify")
+	defer f.Close()
+	dir, base := filepath.Split(p.SettingsFile)
+	if _, err := syscall.InotifyAddWatch(fd, filepath.Clean(dir), syscall.IN_CLOSE_WRITE|syscall.IN_MOVED_TO|syscall.IN_CREATE); err != nil {
+		p.logf("the settings file %s cannot be watched, so a refill of it wakes the puller only at another event: %v", p.SettingsFile, err)
+		return
+	}
+	go func() { <-ctx.Done(); f.Close() }()
+	buf := make([]byte, 4096)
+	for {
+		n, err := f.Read(buf)
+		if err != nil {
+			return
+		}
+		hit := false
+		for off := 0; off+16 <= n; {
+			l := int(binary.LittleEndian.Uint32(buf[off+12:]))
+			name := buf[off+16 : off+16+l]
+			for i, c := range name {
+				if c == 0 {
+					name = name[:i]
+					break
+				}
+			}
+			if string(name) == base {
+				hit = true
+			}
+			off += 16 + l
+		}
+		if hit {
+			p.Wake()
+		}
+	}
+}