--- a/mercurial/debugcommands.py	Sat Apr 01 17:12:48 2017 +0900
+++ b/mercurial/debugcommands.py	Fri Dec 15 09:30:16 2017 +0100
@@ -69,6 +69,7 @@
     templater,
     treediscovery,
     upgrade,
+    url as urlmod,
     util,
     vfs as vfsmod,
 )
@@ -786,6 +787,30 @@
     localrevs = opts['rev']
     doit(localrevs, remoterevs)
 
+_chunksize = 4 << 10
+
+@command('debugdownload',
+    [
+        ('o', 'output', '', _('path')),
+    ],
+    norepo=True)
+def debugdownload(ui, url, output=None, **opts):
+    """download a resource using Mercurial logic and config
+    """
+    fh = urlmod.open(ui, url, output)
+
+    dest = ui
+    if output:
+        dest = open(output, "wb", _chunksize)
+    try:
+        data = fh.read(_chunksize)
+        while data:
+            dest.write(data)
+            data = fh.read(_chunksize)
+    finally:
+        if output:
+            dest.close()
+
 @command('debugextensions', cmdutil.formatteropts, [], norepo=True)
 def debugextensions(ui, **opts):
     '''show information about active extensions'''

--- a/tests/test-completion.t	Sat Apr 01 17:12:48 2017 +0900
+++ b/tests/test-completion.t	Fri Dec 15 09:30:16 2017 +0100
@@ -85,6 +85,7 @@
   debugdeltachain
   debugdirstate
   debugdiscovery
+  debugdownload
   debugextensions
   debugfileset
   debugformat
@@ -263,6 +264,7 @@
   debugdeltachain: changelog, manifest, dir, template
   debugdirstate: nodates, datesort
   debugdiscovery: old, nonheads, rev, ssh, remotecmd, insecure
+  debugdownload: output
   debugextensions: template
   debugfileset: rev
   debugformat: template

--- a/tests/test-help.t	Sat Apr 01 17:12:48 2017 +0900
+++ b/tests/test-help.t	Fri Dec 15 09:30:16 2017 +0100
@@ -919,6 +919,8 @@
                  show the contents of the current dirstate
    debugdiscovery
                  runs the changeset discovery protocol in isolation
+   debugdownload
+                 download a resource using Mercurial logic and config
    debugextensions
                  show information about active extensions
    debugfileset  parse and apply a fileset specification

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-url-download.t	Fri Dec 15 09:30:16 2017 +0100
@@ -0,0 +1,36 @@
+#require serve
+
+  $ hg init server
+  $ hg serve -R server -p $HGPORT -d --pid-file=hg1.pid -E ../error.log
+  $ cat hg1.pid >> $DAEMON_PIDS
+
+Check basic fetching
+
+  $ hg debugdownload "http://localhost:$HGPORT/?cmd=lookup&key=tip"
+  1 0000000000000000000000000000000000000000
+  $ hg debugdownload  -o null.txt "http://localhost:$HGPORT/?cmd=lookup&key=null"
+  $ cat null.txt
+  1 0000000000000000000000000000000000000000
+
+Check the request is made from the usual Mercurial logic
+(rev details, give different content if the request has a Mercurial user agent)
+
+  $ get-with-headers.py --headeronly "localhost:$HGPORT" "rev/tip" content-type
+  200 Script output follows
+  content-type: text/html; charset=ascii
+  $ hg debugdownload "http://localhost:$HGPORT/rev/tip"
+  
+  # HG changeset patch
+  # User 
+  # Date 0 0
+  # Node ID 0000000000000000000000000000000000000000
+  
+  
+  
+  
+
+Check other kind of compatible url
+
+  $ hg debugdownload ./null.txt
+  1 0000000000000000000000000000000000000000
+