[PATCH 1 of 2] verify: document corner cases

[Jun Wu]

# HG changeset patch
# User Jun Wu <quark at fb.com>
# Date 1490823901 25200
#      Wed Mar 29 14:45:01 2017 -0700
# Node ID 1f7890370b5437466534cbd0a313d21671dade03
# Parent  e9fda3b8614a8b701bd48041afa8b709e1227f27
# Available At https://bitbucket.org/quark-zju/hg-draft
#              hg pull https://bitbucket.org/quark-zju/hg-draft -r 1f7890370b54
verify: document corner cases

It seems a good idea to list all kinds of "surprises" and expected behavior
to make the upcoming changes easier to understand.

Note: the comment added do not reflect the actual behavior of the current
code.

diff --git a/mercurial/verify.py b/mercurial/verify.py
--- a/mercurial/verify.py
+++ b/mercurial/verify.py
@@ -381,4 +381,44 @@ class verifier(object):
 
                 # verify contents
+                #
+                # Define 4 cases to help understand corner cases:
+                #
+                #                       | common | rename | meta  | ext
+                #  -------------------------------------------------------
+                #   flags()             | 0      | 0      | 0     | not 0
+                #   renamed()           | False  | True   | False | ?
+                #   rawtext[0:2]=='\1\n'| False  | True   | True  | ?
+                #
+                # "rawtext" means the raw text stored in revlog data, which
+                # could be retrieved by "revision(rev, raw=True)". "text"
+                # mentioned below is "revision(rev, raw=False)".
+                #
+                # There are 3 different lengths stored physically:
+                #  1. L1: rawsize, stored in revlog index
+                #  2. L2: len(rawtext), stored in revlog data
+                #  3. L3: len(text), stored in revlog data if flags=0, or
+                #     possibly somewhere else if flags!=0
+                #
+                # L1 should be equal to L2. L3 is unrelated to L1 or L2. But
+                # text may or may not affect commit hash depending on flag
+                # processors (see revlog.addflagprocessor).
+                #
+                #              | common  | rename | meta  | ext
+                # -------------------------------------------------
+                #    rawsize() | L1      | L1     | L1    | L1
+                #       size() | L1      | L2-LM  | L1(*) | L1 (?)
+                # len(rawtext) | L2      | L2     | L2    | L2
+                #    len(text) | L2      | L2     | L2    | L3
+                #  len(read()) | L2      | L2-LM  | L2-LM | L3
+                #
+                # LM:  length of metadata, depending on rawtext
+                # (*): not ideal, see comment in filelog.size
+                # (?): could be "- len(meta)" if the resolved content has
+                #      rename metadata
+                #
+                # Checks needed to be done:
+                #  1. length check: L1 == L2, in all cases.
+                #  2. hash check: depending on flag processor, we may need to
+                #     use either "text" (external), or "rawtext" (in revlog).
                 try:
                     l = len(fl.read(n))