From ff7e068bf718545fa594481e62f4fc5537c583f8 Mon Sep 17 00:00:00 2001
From: Julien Lepiller <julien@lepiller.eu>
Date: Sun, 9 Feb 2020 19:47:27 +0100
Subject: guix: download: Add partial download support.

* nix/libstore/build.cc (tryToBuild): Do not remove invalid fixed-output
derivations.
* guix/build/download.scm (http-fetch): Add a range argument.
(url-fetch): Performa partial download if a file already exists.
---
 guix/build/download.scm | 36 +++++++++++++++++++++++++++---------
 nix/libstore/build.cc   |  1 +
 2 files changed, 28 insertions(+), 9 deletions(-)

diff --git a/guix/build/download.scm b/guix/build/download.scm
index 0f2d5f402a..c2b710becc 100644
--- a/guix/build/download.scm
+++ b/guix/build/download.scm
@@ -654,11 +654,13 @@ Return the resulting target URI."
                     #:query    (uri-query    ref)
                     #:fragment (uri-fragment ref)))))
 
-(define* (http-fetch uri #:key timeout (verify-certificate? #t))
+(define* (http-fetch uri #:key timeout (verify-certificate? #t) range)
   "Return an input port containing the data at URI, and the expected number of
 bytes available or #f.  When TIMEOUT is true, bail out if the connection could
 not be established in less than TIMEOUT seconds.  When VERIFY-CERTIFICATE? is
-true, verify HTTPS certificates; otherwise simply ignore them."
+true, verify HTTPS certificates; otherwise simply ignore them.  When RANGE is
+a number, it is the number of bytes we want to skip from the data at URI;
+otherwise the full document is requested."
 
   (define headers
     `(;; Some web sites, such as http://dist.schmorp.de, would block you if
@@ -670,6 +672,10 @@ true, verify HTTPS certificates; otherwise simply ignore them."
       ;; Acceptable" when not explicitly told that everything is accepted.
       (Accept . "*/*")
 
+      ,@(if range
+            `((Range . ,(string-append "bytes=" (number->string range) "-")))
+            '())
+
       ;; Basic authentication, if needed.
       ,@(match (uri-userinfo uri)
           ((? string? str)
@@ -690,7 +696,8 @@ true, verify HTTPS certificates; otherwise simply ignore them."
                 ((code)
                  (response-code resp)))
     (case code
-      ((200)                                      ; OK
+      ((200                                       ; OK
+        206)                                      ; Partial content
        (values port (response-content-length resp)))
       ((301                                       ; moved permanently
         302                                       ; found (redirection)
@@ -703,7 +710,8 @@ true, verify HTTPS certificates; otherwise simply ignore them."
          (close connection)
          (http-fetch uri
                      #:timeout timeout
-                     #:verify-certificate? verify-certificate?)))
+                     #:verify-certificate? verify-certificate?
+                     #:range range)))
       (else
        (error "download failed" (uri->string uri)
               code (response-reason-phrase resp))))))
@@ -775,10 +783,15 @@ otherwise simply ignore them."
       ((http https)
        (false-if-exception*
         (let-values (((port size)
-                      (http-fetch uri
-                                  #:verify-certificate? verify-certificate?
-                                  #:timeout timeout)))
-          (call-with-output-file file
+                      (if (file-exists? file)
+                        (http-fetch uri
+                                    #:verify-certificate? verify-certificate?
+                                    #:timeout timeout
+                                    #:range (stat:size (stat file)))
+                        (http-fetch uri
+                                    #:verify-certificate? verify-certificate?
+                                    #:timeout timeout))))
+          (call-with-port (open-file file (if (file-exists? file) "a" "w"))
             (lambda (output)
               (dump-port* port output
                           #:buffer-size %http-receive-buffer-size
@@ -817,7 +830,12 @@ otherwise simply ignore them."
   (let try ((uri (append uri content-addressed-uris)))
     (match uri
       ((uri tail ...)
-       (or (fetch uri file)
+       (or (if (file-exists? file)
+	       ;; If the file already exists, we do a partial fetch for the
+	       ;; remainder.  If something went wrong, we remove the file
+	       ;; and try to fetch the whole file instead.
+               (or (fetch uri file) (begin (delete-file file) (fetch uri file)))
+               (fetch uri file))
            (try tail)))
       (()
        (format (current-error-port) "failed to download ~s from ~s~%"
diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index 17e92c68a7..176ab40226 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -1320,6 +1320,7 @@ void DerivationGoal::tryToBuild()
         Path path = i->second.path;
         if (worker.store.isValidPath(path)) continue;
         if (!pathExists(path)) continue;
+	if (fixedOutput) continue;
         debug(format("removing invalid path `%1%'") % path);
         deletePath(path);
     }
-- 
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