Improve submodule subscription documentation

Current documentation does not cover some limitations in the
implementation. This might cause a Gerrit user to spend valuable time
trying to troubleshoot why a submodule subscription is not working as
expected. Examples of this are:

 * Not possible to use the feature with SSH hostname different from
   canonical web URL hostname
 * Relative submodules must contain the full Gerrit project name

Change-Id: I5acf608b366f3f64047730c2b691b1e6bf62a2ea

1 files changed, 74 insertions, 28 deletions

diff --git a/Documentation/user-submodules.txt b/Documentation/user-submodules.txt
index cfaf3e981d..6aab43fc11 100644
--- a/Documentation/user-submodules.txt
+++ b/Documentation/user-submodules.txt
@@ -1,5 +1,5 @@
-Gerrit Code Review - Superprojects subscribed to submodules updates
-===================================================================
+Gerrit Code Review - Superproject subscription to submodules updates
+====================================================================
 
 Description
 -----------
@@ -25,7 +25,7 @@ commit having the updated gitlinks.
 Git Submodules Overview
 -----------------------
 
-It is a git feature that allows an external repository to be
+Submodules are a git feature that allows an external repository to be
 attached inside a repository at a specific path. The objective here
 is to provide a brief overview, further details can be found
 in the official git submodule command documentation.
@@ -37,20 +37,20 @@ at path 'a' by executing the following command when being inside
 'super':
 =====
 git submodule add ssh://server/a a
-====
+=====
 
 Still considering the above example, after its execution notice that
 inside the local repository 'super' the 'a' folder is considered a
 gitlink to the external repository 'a'. Also notice a file called
-.gitmodules is created (it is a config file containing the
-subscription of 'a'). To provide the sha-1 each gitlink points to in
+.gitmodules is created (it is a configuration file containing the
+subscription of 'a'). To provide the SHA-1 each gitlink points to in
 the external repository, one should use the command:
 ====
 git submodule status
 ====
 
 In the example provided, if 'a' is updated and 'super' is supposed
-to see the latest sha-1 (considering here 'a' has only the master
+to see the latest SHA-1 (considering here 'a' has only the master
 branch), one should then commit the modified gitlink for 'a' in
 the 'super' project. Actually it would not even need to be an
 external update, one could move to 'a' folder (insider 'super'),
@@ -63,11 +63,11 @@ Creating a New Subscription
 Defining the Submodule Branch
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-This is required because Submodule subscription is actually the
+This is required because submodule subscription is actually the
 subscription of a submodule project and one of its branches for
 a branch of a super project.
 
-Since it manages subscriptions in the branch scope, we could have
+Since Gerrit manages subscriptions in the branch scope, we could have
 a scenario having a project called 'super' having a branch 'integration'
 subscribed to a project called 'a' in branch 'integration', and also
 having the same 'super' project but in branch 'dev' subscribed to the 'a'
@@ -77,22 +77,23 @@ After adding the git submodule to a super project, one should edit
 the .gitmodules file to add a branch field to each submodule
 section which is supposed to be subscribed.
 
-The branch field is not filled by the git submodule command. Its value
-should indicate the branch of a submodule project that when updated
-will trigger automatic update of its registered gitlink.
+As the branch field is a Gerrit specific field it will not be filled
+automatically by the git submodule command, so one needs to edit it
+manually. Its value should indicate the branch of a submodule project
+that when updated will trigger automatic update of its registered
+gitlink.
 
-The branch value could be '.' if the submodule project branch
+The branch value could be "'.'" if the submodule project branch
 has the same name as the destination branch of the commit having
 gitlinks/.gitmodules file.
 
-The branch field of a submodule section is a custom git submodule
-feature for Gerrit use. One should always be sure to fill it in
-editing .gitmodules file after adding submodules to a super project,
-if it is the intention to make use of the Gerrit feature introduced here.
+If the intention is to make use of the Gerrit feature described
+here, one should always be sure to update the .gitmodules file after
+adding submodules to a super project.
 
-Any git submodules which are added and not have the branch field
-available in the .gitmodules file will not be subscribed by Gerrit
-to automatically update the superproject.
+If a git submodule is added but the branch field is not added to the
+.gitmodules file, Gerrit will not create a subscription for the
+submodule and there will be no automatic updates to the superproject.
 
 Detecting and Subscribing Submodules
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -118,17 +119,62 @@ is merged in branch 'dev-of-a' of 'a' project, Gerrit automatically
 creates a new commit on branch 'dev' of 'super' updating the gitlink
 to point to the just merged commit.
 
-Canonical Web Url
-~~~~~~~~~~~~~~~~~
+Subscription Limitations
+~~~~~~~~~~~~~~~~~~~~~~~~
 
-Gerrit will automatically update only the superprojects that added
-the submodules of urls of the running server (the one described in
-the canonical web url value in Gerrit configuration file).
+Gerrit will only automatically update superprojects where the
+submodules are hosted on the same Gerrit instance as the
+superproject. Gerrit determines this by checking the hostname of the
+submodule specified in the .gitmodules file and comparing it to the
+hostname from the canonical web URL.
+
+It is currently not possible to use the submodule subscription feature
+with a canonical web URL hostname that differs from the hostname of
+the submodule. Instead relative submodules should be used.
 
 The Gerrit instance administrator group should always certify to
-provide the canonical web url value in its configuration file. Users
-should certify to use the url value of the running Gerrit instance to
-add/subscribe submodules.
+provide the canonical web URL value in its configuration file. Users
+should certify to use the correct hostname of the running Gerrit
+instance to add/subscribe submodules.
+
+Relative submodules
+~~~~~~~~~~~~~~~~~~~
+
+To enable easier usage of Gerrit mirrors and/or distribution over
+several protocols, such as plain git and HTTP(S) as well as SSH, one
+can use relative submodules. This means that instead of providing the
+entire URL to the submodule a relative path is stated in the
+.gitmodules file.
+
+Gerrit will try to match the entire project name of the submodule
+including directories. Therefore it is important to supply the full
+path name of the Gerrit project, not only relative to the super
+repository. See the following example:
+
+We have a super repository placed under a sub directory.
+
+  product/super_repository.git
+
+To this repository we wish add a submodule "deeper" into the directory
+structure.
+
+  product/framework/subcomponent.git
+
+Now we need to edit the .gitmodules to include the complete path to
+the Gerrit project. Observe that we need to use two "../" to include
+the complete Gerrit project path.
+
+  path = subcomponent.git
+  url = ../../product/framework/subcomponent.git
+  branch = master
+
+In contrast the following will not setup proper submodule
+subscription, even if the submodule will be successfully cloned by git
+from Gerrit.
+
+  path = subcomponent.git
+  url = ../framework/subcomponent.git
+  branch = master
 
 Removing Subscriptions
 ----------------------