Python memory snapshot: add magic number and version

src/pyodide/internal/python.js

@@ -419,20 +419,27 @@ function setUploadFunction(toUpload) {
   };
 }
 
+// "\x00snp"
+const SNAPSHOT_MAGIC = 0x6d736100;
+const SNAPSHOT_VERSION = 1;
+
 /**
  * Encode heap and dsoJSON into the memory snapshot artifact that we'll upload
  */
 function encodeSnapshot(heap, dsoJSON) {
   const dsoString = JSON.stringify(dsoJSON);
-  let sx = 8 + 2 * dsoString.length;
+  let snapshotOffset = 8 + 2 * dsoString.length;
   // align to 8 bytes
-  sx = Math.ceil(sx / 8) * 8;
-  const toUpload = new Uint8Array(sx + heap.length);
+  snapshotOffset = Math.ceil(snapshotOffset / 8) * 8;
+  const toUpload = new Uint8Array(snapshotOffset + heap.length);
   const encoder = new TextEncoder();
   const { written } = encoder.encodeInto(dsoString, toUpload.subarray(8));
+  const jsonLength = written;
   const uint32View = new Uint32Array(toUpload.buffer);
-  uint32View[0] = sx;
-  uint32View[1] = written;
+  uint32View[0] = SNAPSHOT_MAGIC;
+  uint32View[1] = SNAPSHOT_VERSION;
+  uint32View[2] = snapshotOffset;
+  uint32View[3] = jsonLength;
   toUpload.subarray(sx).set(heap);
   return toUpload;
 }
@@ -442,8 +449,13 @@ function encodeSnapshot(heap, dsoJSON) {
  */
 function decodeSnapshot(memorySnapshot) {
   const uint32View = new Uint32Array(memorySnapshot);
-  const snapshotOffset = uint32View[0];
-  const jsonLength = uint32View[1];
+  if (uint32View[0] !== SNAPSHOT_MAGIC) {
+    throw new Error("Memory snapshot doesn't start with snapshot magic number");
+  }
+  const snapshotVersion = uint32View[1];
+  // Currently we don't do anything with snapshotVersion.
+  const snapshotOffset = uint32View[2];
+  const jsonLength = uint32View[3];
   const jsonView = new Uint8Array(memorySnapshot, 8, jsonLength);
   DSO_METADATA = JSON.parse(new TextDecoder().decode(jsonView));
   MEMORY = new Uint8Array(memorySnapshot, snapshotOffset);