git: rely on msmtp for smtp if msmtp is enabled (#1829)

If a user using msmtp to send all their email, it would be preferred if
git used it as well.

The only settings necessary are to set the smtp server to the msmtp
binary and set envelop sender to true, which makes git call msmtp with
the -f flag to set the from address from the email.

modules/misc/news.nix

@@ -1863,6 +1863,15 @@ in
           lists to polybar-style 'foo-0, foo-1, ...' lists.
         '';
       }
+      {
+        time = "2021-02-25T22:36:43+00:00";
+        condition = config.programs.git.enable
+          && any config.accounts.email.accounts (account: account.msmtp.enable);
+        message = ''
+          Git will now defer to msmtp for sending emails if
+          'accounts.email.accounts.<name>.msmtp.enable' is true.
+        '';
+      }
     ];
   };
 }

modules/programs/git.nix

@@ -276,7 +276,12 @@ in {
 
         genIdentity = name: account:
           with account;
-          nameValuePair "sendemail.${name}" ({
+          nameValuePair "sendemail.${name}" (if account.msmtp.enable then {
+            smtpServer = "${pkgs.msmtp}/bin/msmtp";
+            envelopeSender = true;
+            from = address;
+          } else
+            {
               smtpEncryption = if smtp.tls.enable then
                 (if smtp.tls.useStartTls
                 || versionOlder config.home.stateVersion "20.09" then

tests/modules/programs/git/default.nix

@@ -1,5 +1,6 @@
 {
   git-with-email = ./git-with-email.nix;
   git-with-most-options = ./git.nix;
+  git-with-msmtp = ./git-with-msmtp.nix;
   git-with-str-extra-config = ./git-with-str-extra-config.nix;
 }

tests/modules/programs/git/git-with-msmtp-expected.conf

@@ -0,0 +1,14 @@
+[sendemail "hm-account"]
+	from = "hm@example.org"
+	smtpEncryption = "tls"
+	smtpServer = "smtp.example.org"
+	smtpUser = "home.manager.jr"
+
+[sendemail "hm@example.com"]
+	envelopeSender = true
+	from = "hm@example.com"
+	smtpServer = "@msmtp@/bin/msmtp"
+
+[user]
+	email = "hm@example.com"
+	name = "H. M. Test"

tests/modules/programs/git/git-with-msmtp.nix

@@ -0,0 +1,45 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  imports = [ ../../accounts/email-test-accounts.nix ];
+
+  config = {
+    accounts.email.accounts."hm@example.com".msmtp.enable = true;
+    programs.git = {
+      enable = true;
+      package = pkgs.gitMinimal;
+      userEmail = "hm@example.com";
+      userName = "H. M. Test";
+    };
+
+    home.stateVersion = "20.09";
+
+    nmt.script = ''
+      function assertGitConfig() {
+        local value
+        value=$(${pkgs.gitMinimal}/bin/git config \
+          --file $TESTED/home-files/.config/git/config \
+          --get $1)
+        if [[ $value != $2 ]]; then
+          fail "Expected option '$1' to have value '$2' but it was '$value'"
+        fi
+      }
+
+      assertFileExists home-files/.config/git/config
+      assertFileContent home-files/.config/git/config \
+                        ${
+                          pkgs.substituteAll {
+                            inherit (pkgs) msmtp;
+                            src = ./git-with-msmtp-expected.conf;
+                          }
+                        }
+
+      assertGitConfig "sendemail.hm@example.com.from" "hm@example.com"
+      assertGitConfig "sendemail.hm-account.from" "hm@example.org"
+      assertGitConfig "sendemail.hm@example.com.smtpServer" "${pkgs.msmtp}/bin/msmtp"
+      assertGitConfig "sendemail.hm@example.com.envelopeSender" "true"
+    '';
+  };
+}