Add the .id() method.

README.rst

 ..
 .. NB:  This file is machine generated, DO NOT EDIT!
 ..
-.. Edit vmod.vcc and run make instead
+.. Edit ./vmod_file.vcc and run make instead
 ..
 
 .. role:: ref(emphasis)
 
 =========
-VMOD file
+vmod_file
 =========
 
 -----------------------------------------------------------------
@@ -452,6 +452,13 @@ Example::
   set resp.http.Cache-Control = "public, max-age="
 	+ std.integer(duration=rdr.next_check());
 
+.. _xreader.id():
+
+BLOB xreader.id()
+-----------------
+
+XXX ...
+
 .. _file.version():
 
 STRING version()

src/tests/info.vtc

@@ -178,3 +178,37 @@ client c1 {
 	expect resp.http.Error == "false"
 	expect resp.http.Deleted == "true"
 } -run
+
+shell {touch ${tmpdir}/id}
+
+varnish v1 -vcl {
+	import ${vmod_file};
+	import blob;
+	backend b None;
+
+	sub vcl_init {
+		new rdr = file.reader("${tmpdir}/id", ttl=0.1s);
+	}
+
+	sub vcl_recv {
+		return (synth(200));
+	}
+
+	sub vcl_synth {
+		set resp.http.ETag = blob.encode(BASE64, blob=rdr.id());
+		return (deliver);
+	}
+}
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.http.ETag ~ {^[[:alnum:]+/]+=*$}
+} -run
+
+shell {touch ${tmpdir}/id}
+delay .1
+
+# Check the log to verify that ETag changes.
+client c1 -run

src/vmod_file.c

@@ -58,6 +58,9 @@
 /* Other VMODs may check the result of .blob() for this value, see vrt.h */
 #define VMOD_FILE_BLOB_MAGIC 0x069392c4
 
+/* For the result of .id() */
+#define VMOD_FILE_ID_MAGIC 0x001122d7
+
 #define INIT_SLEEP_INTERVAL 0.001
 #define ERRMSG_LEN 128
 #define NO_ERR ("No error")
@@ -663,6 +666,59 @@ vmod_reader_next_check(VRT_CTX, struct VPFX(file_reader) *rdr)
 	return (t.it_value.tv_sec + 1e-9 * t.it_value.tv_nsec);
 }
 
+/*
+ * Note that the contents of the BLOB returned from .id() are
+ * undocumented, any may change in any new version.
+ */
+VCL_BLOB
+vmod_reader_id(VRT_CTX, struct VPFX(file_reader) *rdr)
+{
+	time_t secs;
+	long nsecs;
+	dev_t dev;
+	ino_t ino;
+	uintptr_t snap;
+	struct vrt_blob *blob;
+	char *data;
+
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	CHECK_OBJ_NOTNULL(ctx->ws, WS_MAGIC);
+	CHECK_OBJ_NOTNULL(rdr, FILE_READER_MAGIC);
+	CHECK_OBJ_NOTNULL(rdr->info, FILE_INFO_MAGIC);
+
+	AZ(pthread_rwlock_rdlock(&rdr->lock));
+	ERRCHK(ctx, rdr, "rdr", NULL);
+	secs = rdr->info->mtime.tv_sec;
+	nsecs = rdr->info->mtime.tv_nsec;
+	dev = rdr->info->dev;
+	ino = rdr->info->ino;
+	AZ(pthread_rwlock_unlock(&rdr->lock));
+
+	snap = WS_Snapshot(ctx->ws);
+	if ((blob = WS_Alloc(ctx->ws, sizeof(*blob))) == NULL) {
+		VFAIL(ctx, "%s.id(): insufficient workspace for blob structure",
+		      rdr->obj_name);
+		return NULL;
+	}
+	if ((data = WS_Alloc(ctx->ws, sizeof(secs) + sizeof(nsecs)
+			     + sizeof(dev) + sizeof(ino))) == NULL) {
+		WS_Reset(ctx->ws, snap);
+		VFAIL(ctx, "%s.id(): insufficient workspace for blob data",
+		      rdr->obj_name);
+		return NULL;
+	}
+
+	blob->type = VMOD_FILE_ID_MAGIC;
+	blob->len = sizeof(secs) + sizeof(nsecs) + sizeof(dev) + sizeof(ino);
+	blob->blob = data;
+	memcpy(data, &secs, sizeof(secs));
+	memcpy(data + sizeof(secs), &nsecs, sizeof(nsecs));
+	memcpy(data + sizeof(secs) + sizeof(nsecs), &dev, sizeof(dev));
+	memcpy(data + sizeof(secs) + sizeof(nsecs) + sizeof(dev), &ino,
+	       sizeof(ino));
+	return (blob);
+}
+
 VCL_STRING
 vmod_version(VRT_CTX)
 {

src/vmod_file.vcc

@@ -411,6 +411,10 @@ Example::
   set resp.http.Cache-Control = "public, max-age="
 	+ std.integer(duration=rdr.next_check());
 
+$Method BLOB .id()
+
+XXX ...
+
 $Function STRING version()
 
 Return the version string for this VMOD.