pmb.helpers.git: parse channels.cfg (MR 1912)

Prepare to base postmarketOS on Alpine stable by parsing the new
channels.cfg file in pmaports.git, that describes which channel
needs which branches and mirror dirs from postmarketOS and Alpine.

Use the information in pmb.helpers.git.get_branches_official() first,
more is coming in follow-up commits.

Read the file from origin/master, so we get the latest fetched version
even if the last checked out master branch is not up-to-date (think of
currently checked out release branch instead of master, master will
never be updated to point to latest origin/master). Allow to override
the file with a new --config-channels parameter.

Related: https://postmarketos.org/channels.cfg

pmb/helpers/git.py

@@ -1,5 +1,6 @@
 # Copyright 2020 Oliver Smith
 # SPDX-License-Identifier: GPL-3.0-or-later
+import configparser
 import logging
 import os
 import time
@@ -100,12 +101,71 @@ def get_upstream_remote(args, name_repo):
                        " repository: {}".format(name_repo, url, path))
 
 
+def parse_channels_cfg(args):
+    """ Parse channels.cfg from pmaports.git, origin/master branch.
+        Reference: https://postmarketos.org/channels.cfg
+        :returns: dict like: {"meta": {"recommended": "edge"},
+                              "channels": {"edge": {"description": ...,
+                                                    "branch_pmaports": ...,
+                                                    "branch_aports": ...,
+                                                    "mirrordir_alpine": ...},
+                                           ...}} """
+    # Cache during one pmbootstrap run
+    cache_key = "pmb.helpers.git.parse_channels_cfg"
+    if args.cache[cache_key]:
+        return args.cache[cache_key]
+
+    # Read with configparser
+    cfg = configparser.ConfigParser()
+    if args.config_channels:
+        cfg.read([args.config_channels])
+    else:
+        remote = get_upstream_remote(args, "pmaports")
+        command = ["git", "show", f"{remote}/master:channels.cfg"]
+        stdout = pmb.helpers.run.user(args, command, args.aports,
+                                      output_return=True, check=False)
+        try:
+            cfg.read_string(stdout)
+        except configparser.MissingSectionHeaderError:
+            logging.info("NOTE: fix this by fetching your pmaports.git, e.g."
+                         " with 'pmbootstrap pull'")
+            raise RuntimeError("Failed to read channels.cfg from"
+                               f" '{remote}/master' branch of your local"
+                               " pmaports clone")
+
+    # Meta section
+    ret = {"channels": {}}
+    ret["meta"] = {"recommended": cfg.get("channels.cfg", "recommended")}
+
+    # Channels
+    for channel in cfg.sections():
+        if channel == "channels.cfg":
+            continue  # meta section
+
+        ret["channels"][channel] = {}
+        for key in ["description", "branch_pmaports", "branch_aports",
+                    "mirrordir_alpine"]:
+            value = cfg.get(channel, key)
+            ret["channels"][channel][key] = value
+
+    args.cache[cache_key] = ret
+    return ret
+
+
 def get_branches_official(args, name_repo):
     """ Get all branches that point to official release channels.
         :returns: list of supported branches, e.g. ["master", "3.11"] """
-    # More sophisticated logic to figure out the branches will be added soon:
-    # https://gitlab.com/postmarketOS/postmarketos/issues/11
-    return ["master"]
+    # This functions gets called with pmaports and aports_upstream, because
+    # both are displayed in "pmbootstrap status". But it only makes sense
+    # to display pmaports there, related code will be refactored soon (#1903).
+    if name_repo != "pmaports":
+        return ["master"]
+
+    channels_cfg = parse_channels_cfg(args)
+    ret = []
+    for channel, channel_data in channels_cfg["channels"].items():
+        ret.append(channel_data["branch_pmaports"])
+    return ret
 
 
 def pull(args, name_repo):