dispatch: add HGPLAIN=+strictflags to restrict early parsing of global options If this feature is enabled, early options are parsed using the global options table. As the parser stops processing options when non/unknown option is encountered, it won't mistakenly take an option value as a new early option. Still "--" can be injected to terminate the parsing (e.g. "hg -R -- log"), I think it's unlikely to lead to an RCE. To minimize a risk of this change, new fancyopts.earlygetopt() path is enabled only when +strictflags is set. Also the strict parser doesn't support '--repo', a short for '--repository' yet. This limitation will be removed later. As this feature is backward incompatible, I decided to add a new opt-in mechanism to HGPLAIN. I'm not pretty sure if this is the right choice, but I'm thinking of adding +feature/-feature syntax to HGPLAIN. Alternatively, we could add a new environment variable. Any bikeshedding is welcome. Note that HGPLAIN=+strictflags doesn't work correctly in chg session since command arguments are pre-processed in C. This wouldn't be easily fixed.

--- a/mercurial/help/environment.txt	Thu Nov 23 22:04:53 2017 +0900
+++ b/mercurial/help/environment.txt	Thu Nov 23 22:17:03 2017 +0900
@@ -56,9 +56,17 @@
     localization. This can be useful when scripting against Mercurial
     in the face of existing user configuration.
 
+    In addition to the features disabled by ``HGPLAIN=``, the following
+    values can be specified to adjust behavior:
+
+    ``+strictflags``
+        Restrict parsing of command line flags.
+
     Equivalent options set via command line flags or environment
     variables are not overridden.
 
+    See :hg:`help scripting` for details.
+
 HGPLAINEXCEPT
     This is a comma-separated list of features to preserve when
     HGPLAIN is enabled. Currently the following values are supported: