--- a/mercurial/streamclone.py	Tue Oct 01 15:55:49 2024 +0200
+++ b/mercurial/streamclone.py	Tue Oct 01 16:07:51 2024 +0200
@@ -567,53 +567,113 @@
 
 
 class VolatileManager:
-    """Manage temporary backup of volatile file during stream clone
+    """Manage temporary backups of volatile files during stream clone.
 
-    This should be used as a Python context, the copies will be discarded when
-    exiting the context.
+    This class will keep open file handles for the volatile files, writing the
+    smaller ones on disk if the number of open file handles grow too much.
 
-    A copy can be done by calling the object on the real path (encoded full
-    path)
+    This should be used as a Python context, the file handles and copies will
+    be discarded when exiting the context.
 
-    The backup path can be retrieved using the __getitem__ protocol, obj[path].
-    On file without backup, it will return the unmodified path. (equivalent to
-    `dict.get(x, x)`)
+    The preservation can be done by calling the object on the real path
+    (encoded full path).
+
+    Valid filehandles for any file should be retrieved by calling `open(path)`.
     """
 
+    # arbitrarily picked as "it seemed fine" and much higher than the current
+    # usage.
+    MAX_OPEN = 100
+
     def __init__(self):
+        self._counter = 0
+        self._volatile_fps = None
         self._copies = None
         self._dst_dir = None
 
     def __enter__(self):
-        if self._copies is not None:
-            msg = "Copies context already open"
-            raise error.ProgrammingError(msg)
+        if self._counter == 0:
+            assert self._volatile_fps is None
+            self._volatile_fps = {}
+        self._counter += 1
+        return self
+
+    def __exit__(self, *args, **kwars):
+        """discard all backups"""
+        self._counter -= 1
+        if self._counter == 0:
+            for _size, fp in self._volatile_fps.values():
+                fp.close()
+            self._volatile_fps = None
+            if self._copies is not None:
+                for tmp in self._copies.values():
+                    util.tryunlink(tmp)
+                util.tryrmdir(self._dst_dir)
+            self._copies = None
+            self._dst_dir = None
+            assert self._volatile_fps is None
+            assert self._copies is None
+            assert self._dst_dir is None
+
+    def _init_tmp_copies(self):
+        """prepare a temporary directory to save volatile files
+
+        This will be used as backup if we have too many files open"""
+        assert 0 < self._counter
+        assert self._copies is None
+        assert self._dst_dir is None
         self._copies = {}
         self._dst_dir = pycompat.mkdtemp(prefix=b'hg-clone-')
-        return self
+
+    def _flush_some_on_disk(self):
+        """move some of the open files to tempory files on disk"""
+        if self._copies is None:
+            self._init_tmp_copies()
+        flush_count = self.MAX_OPEN // 2
+        for src, (size, fp) in sorted(self._volatile_fps.items())[:flush_count]:
+            prefix = os.path.basename(src)
+            fd, dst = pycompat.mkstemp(prefix=prefix, dir=self._dst_dir)
+            self._copies[src] = dst
+            os.close(fd)
+            # we no longer hardlink, but on the other hand we rarely do this,
+            # and we do it for the smallest file only and not at all in the
+            # common case.
+            with open(dst, 'wb') as bck:
+                fp.seek(0)
+                bck.write(fp.read())
+            del self._volatile_fps[src]
+            fp.close()
+
+    def _keep_one(self, src):
+        """preserve an open file handle for a given path"""
+        # store the file quickly to ensure we close it if any error happens
+        _, fp = self._volatile_fps[src] = (None, open(src, 'rb'))
+        fp.seek(0, os.SEEK_END)
+        size = fp.tell()
+        self._volatile_fps[src] = (size, fp)
 
     def __call__(self, src):
-        """create a backup of the file at src"""
-        prefix = os.path.basename(src)
-        fd, dst = pycompat.mkstemp(prefix=prefix, dir=self._dst_dir)
-        os.close(fd)
-        self._copies[src] = dst
-        util.copyfiles(src, dst, hardlink=True)
-        return dst
+        """preserve the volatile file at src"""
+        assert 0 < self._counter
+        if len(self._volatile_fps) >= (self.MAX_OPEN - 1):
+            self._flush_some_on_disk()
+        self._keep_one(src)
 
     @contextlib.contextmanager
     def open(self, src):
-        actual_path = self._copies.get(src, src)
-        with open(actual_path, 'rb') as fp:
+        assert 0 < self._counter
+        entry = self._volatile_fps.get(src)
+        if entry is not None:
+            _size, fp = entry
+            fp.seek(0)
             yield fp
-
-    def __exit__(self, *args, **kwars):
-        """discard all backups"""
-        for tmp in self._copies.values():
-            util.tryunlink(tmp)
-        util.tryrmdir(self._dst_dir)
-        self._copies = None
-        self._dst_dir = None
+        else:
+            if self._copies is None:
+                actual_path = src
+            else:
+                actual_path = self._copies.get(src, src)
+            with open(actual_path, 'rb') as fp:
+                yield fp
 
 
 def _makemap(repo):