b4: Try to download thread from piem-inboxes URL

If a call to piem-b4-am-from-mid fails to generate the thread for a
message ID via piem-mid-to-thread-functions, b4 is called without a
local mbox.  In this case, b4 tries to download the thread from the
URL specified by b4.midmask in the caller's Git configuration.

That works, but it's inconvenient because the user needs to configure
the URL in two places.  If the current buffer is associated with an
inbox in piem-inboxes, try to download the thread from its :url before
falling back to b4's midmask.

Message-Id: <20200817013343.15615-6-kyle@kyleam.com>

2 files changed, 30 insertions, 4 deletions

diff --git a/piem-b4.el b/piem-b4.el
index 537abed..39d32be 100644
--- a/piem-b4.el
+++ b/piem-b4.el
@@ -47,10 +47,6 @@
 
 ;;;; Internals
 
-;; In many cases, we don't really need b4 to download the mbox for us,
-;; as we already have our own mbox to URL mapping.  Perhaps we should
-;; default to using that, but it should still be an option to use b4
-;; so that we honor its customization/URL resolution.
 (defun piem-b4--get-am-files (mid coderepo args)
   (let* ((outdir (file-name-as-directory
                   (make-temp-file "piem-b4-" t)))
@@ -63,6 +59,21 @@
         (funcall fn)
         (unless (= (point-max) 1)
           (setq local-mbox-p t))))
+    ;; `piem-mid-to-thread-functions' didn't generate an mbox.  Next
+    ;; try to download it from a URL at `piem-inboxes'.  Finally, fall
+    ;; back to b4's configuration.
+    (unless local-mbox-p
+      (when-let ((url (piem-inbox-url))
+                 (mid (piem-mid))
+                 (buffer (condition-case nil
+                             (piem-download-and-decompress
+                              (concat url mid "/t.mbox.gz"))
+                           (user-error nil))))
+        (when (buffer-live-p buffer)
+          (with-current-buffer buffer
+            (write-region nil nil mbox-thread))
+          (kill-buffer buffer)
+          (setq local-mbox-p t))))
     ;; Move to the coderepo so that we pick up any b4 configuration
     ;; from there.
     (apply #'piem-process-call coderepo piem-b4-b4-executable "am"
diff --git a/piem.el b/piem.el
index 029b647..3669cfe 100644
--- a/piem.el
+++ b/piem.el
@@ -390,6 +390,21 @@ buffer."
   (delete-region (point) (point-max))
   (goto-char (point-min)))
 
+(defun piem--decompress-callback (status)
+  (if (plist-get status :error)
+      (kill-buffer (current-buffer))
+    (piem--url-remove-header)
+    (piem--url-decompress)))
+
+(defun piem-download-and-decompress (url)
+  "Retrieve gzipped content at URL and decompress it.
+A buffer with the decompressed content is returned.  A live
+buffer indicates that the request did not result in an error."
+  (unless (piem-check-gunzip)
+    (user-error "gunzip executable not found"))
+  (let ((url-asynchronous nil))
+    (url-retrieve url #'piem--decompress-callback)))
+
 (defun piem--write-mbox-to-maildir ()
   (let ((n-messages 0))
     (while (and (not (eobp))