modifiy MEMFS timestamp handling to support better caching (#893)

emsdk/patches/posixish_memfs.patch

@@ -0,0 +1,38 @@
+Modify MEMFS to update timestamp of parent directory on file creation and removal.
+More POSIXish compliant for a one timestamp fs, behaves like a modification date.
+
+--- a/emsdk/fastcomp/emscripten/src/library_memfs.js    2019-08-19 18:11:40.000000000 +0200
++++ b/emsdk/fastcomp/emscripten/src/library_memfs.js	2020-12-23 12:22:38.000000000 +0100
+@@ -88,6 +88,7 @@
+       // add the new node to the parent
+       if (parent) {
+         parent.contents[name] = node;
++        parent.timestamp = node.timestamp;
+       }
+       return node;
+     },
+@@ -213,12 +214,16 @@
+         }
+         // do the internal rewiring
+         delete old_node.parent.contents[old_node.name];
++        var now = Date.now();
++        old_node.parent.timestamp = now;
+         old_node.name = new_name;
+         new_dir.contents[new_name] = old_node;
++        new_dir.timestamp = now;
+         old_node.parent = new_dir;
+       },
+       unlink: function(parent, name) {
+         delete parent.contents[name];
++        parent.timestamp = Date.now();
+       },
+       rmdir: function(parent, name) {
+         var node = FS.lookupNode(parent, name);
+@@ -226,6 +231,7 @@
+           throw new FS.ErrnoError({{{ cDefine('ENOTEMPTY') }}});
+         }
+         delete parent.contents[name];
++        parent.timestamp = Date.now();
+       },
+       readdir: function(node) {
+         var entries = ['.', '..'];

emsdk/tests/test_memfsmtime.py

@@ -0,0 +1,78 @@
+import subprocess
+from . import common
+
+
+def test_memfsmtime(tmpdir):
+    with tmpdir.as_cwd():
+        with open("main.c", "w") as f:
+            f.write(
+                r"""\
+#include <assert.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include <unistd.h>
+#include <sys/stat.h>
+
+void mysleep(int delta)
+{
+    time_t end = time(NULL) + delta;
+
+    while (time(NULL)<end);
+}
+
+time_t getmtime(char *fname)
+{
+    struct stat buf;
+
+    stat(fname, &buf);
+    return buf.st_mtime;
+}
+
+void writefile(char *fname, char *content)
+{
+    int fd = open(fname, O_CREAT | O_TRUNC | O_WRONLY, 0777);
+
+    write(fd, content, strlen(content));
+    close(fd);
+}
+
+int main(int argc, char *argv[])
+{
+    char tmpdir[64] = "/tmp/tmpXXXXXXX";
+    char fname[64];
+    time_t t0, t1, t2, t3;
+
+    mkdtemp(tmpdir);
+    strcpy(fname, tmpdir);
+    strcat(fname, "/foo.py");
+    t0 = getmtime(tmpdir);
+    mysleep(1);
+    writefile(fname, "bar = 54\n");
+    t1 = getmtime(tmpdir);
+    t2 = getmtime(fname);
+    assert(t1 > t0);
+    assert(t1 == t2);
+    mysleep(1);
+    unlink(fname);
+    t3 = getmtime(tmpdir);
+    assert(t3 > t1);
+}
+"""
+            )
+
+        subprocess.run(
+            [
+                "emcc",
+                "-s",
+                "MAIN_MODULE=1",
+                "main.c",
+            ],
+            check=True,
+            env=common.env,
+        )
+        out = subprocess.run(
+            ["node", "a.out.js"], capture_output=True, check=False, env=common.env
+        )
+        assert out.returncode == 0