changeset 42483:c1c1872d25d1

deltas: skip if projected compressed size does not match text size constraint

If we have a delta, we check constraints against a lower bound estimate of the
resulting compressed delta. We then checks this projected size against the ½ⁿ
size constraints. This allows to exclude potential base candidates before doing
any expensive computation.

This only apply to the intermediate-snapshot case since this constraint only apply to
them.

For some pathological cases of a private repository this step provide a
further performance boost (timing from `hg perfrevlogwrite`):

before:  3.145906 seconds
after:   3.010646 seconds

--- a/mercurial/revlogutils/deltas.py	Mon Jan 21 22:37:30 2019 +0100
+++ b/mercurial/revlogutils/deltas.py	Mon Jan 21 22:46:18 2019 +0100
@@ -960,6 +960,12 @@
                 delta = revinfo.cachedelta[1]
         if delta is None:
             delta = self._builddeltadiff(base, revinfo, fh)
+        # snapshotdept need to be neither None nor 0 level snapshot
+        if revlog.upperboundcomp is not None and snapshotdepth:
+            lowestrealisticdeltalen = len(delta) // revlog.upperboundcomp
+            snapshotlimit = revinfo.textlen >> snapshotdepth
+            if snapshotlimit < lowestrealisticdeltalen:
+                return None
         header, data = revlog.compress(delta)
         deltalen = len(header) + len(data)
         offset = revlog.end(len(revlog) - 1)