Keep each VM on the JDK image file it started with

src/image.ts

@@ -8,17 +8,22 @@ import { listIndex, packedRange, parsePack } from "./browser/http.ts";
 import type { Pack } from "./browser/http.ts";
 import type { Backend, Stat } from "./fs.ts";
 
-// The JDK image is read for every boot class, by every VM in the process, so
-// each image file stays open once opened.
+// The JDK image is read for every boot class, so each image file stays open
+// once opened, shared by the VMs that read it. A VM keeps the descriptor it
+// started with: an upgrade can replace the file while VMs that read its index
+// are still running, and new VMs then open the new file.
 const imageFds = new Map<string, number>();
 export function imageFd(file: string): number {
-  let fd = imageFds.get(file);
-  if (fd === undefined) imageFds.set(file, (fd = fs.openSync(file, "r")));
+  const key = `${file}\0${fs.statSync(file).ino}`;
+  let fd = imageFds.get(key);
+  if (fd === undefined) imageFds.set(key, (fd = fs.openSync(file, "r")));
   return fd;
 }
 
 export class ImageBackend implements Backend {
   readonly writable = false;
+  // this VM's descriptor of each image file
+  private readonly fds = new Map<string, number>();
   private readonly dir: string;
   private readonly pack: Pack;
 
@@ -55,7 +60,8 @@ export class ImageBackend implements Backend {
     const out = new Uint8Array(len);
     let n = 0;
     try {
-      const fd = imageFd(path.join(this.dir, "jdk", at.image));
+      let fd = this.fds.get(at.image);
+      if (fd === undefined) this.fds.set(at.image, (fd = imageFd(path.join(this.dir, "jdk", at.image))));
       while (n < len) {
         const got = fs.readSync(fd, out, n, len - n, at.at + off + n);
         if (got === 0) break;

src/node.ts

@@ -71,10 +71,12 @@ export class NodeBackend implements Backend {
   }
   // Other open files keep their buffered contents even if the host removes them.
   canReadRange(p: string) { return !this.writable && p === "/lib/modules"; }
+  // this VM's descriptor of the JDK image
+  private imageFd: number | undefined;
   readRange(p: string, off: number, len: number): Uint8Array | null {
     const image = p === "/lib/modules" && !this.writable;
     try {
-      const fd = image ? imageFd(this.host(p)) : fs.openSync(this.host(p), "r");
+      const fd = image ? (this.imageFd ??= imageFd(this.host(p))) : fs.openSync(this.host(p), "r");
       try {
         if (!image && !fs.fstatSync(fd).isFile()) return null;
         const out = new Uint8Array(len);

test/image.test.ts

@@ -3,7 +3,7 @@
 // dist/ is built.
 
 import { spawnSync } from "node:child_process";
-import { existsSync, mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { existsSync, mkdirSync, mkdtempSync, renameSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { test } from "node:test";
@@ -65,6 +65,25 @@ test("ImageBackend reads the index, packed files and module images", () => {
   }
 });
 
+test("a VM keeps reading the image it started with when an upgrade replaces it", () => {
+  const dir = mkdtempSync(join(tmpdir(), "jw-image-"));
+  try {
+    mkdirSync(join(dir, "jdk"));
+    writeFileSync(join(dir, "jdk", "image.jar"), "OLD!");
+    writeFileSync(join(dir, "boot.pack"), pack([["/index.txt", "d /lib\nf /lib/x 4 image:0"]]));
+    const text = (d: Uint8Array | null) => d && new TextDecoder().decode(d);
+    const before = new ImageBackend(dir);
+    assert.equal(text(before.read("/lib/x")), "OLD!");
+    // as a package manager replaces a file: a new one renamed over it
+    writeFileSync(join(dir, "jdk", "image.new"), "NEW!");
+    renameSync(join(dir, "jdk", "image.new"), join(dir, "jdk", "image.jar"));
+    assert.equal(text(new ImageBackend(dir).read("/lib/x")), "NEW!");
+    assert.equal(text(before.readRange("/lib/x", 0, 4)), "OLD!");
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
 test("compact index keeps paths, sizes, offsets, and directory order", () => {
   const buf = pack([["/index.prefix", "0 d /lib\n0 f /lib/one 3 image:0\n7 two 4 image:3 javac\n0 d /modules"]]);
   const image = parsePack(buf.buffer as ArrayBuffer);