DRAFT substitute: Add IPFS support.

Missing:

  - documentation
  - command-line options
  - progress report when downloading over IPFS
  - fallback when we fail to fetch from IPFS

* guix/scripts/substitute.scm (<narinfo>)[ipfs]: New field.
(read-narinfo): Read "IPFS".
(process-substitution/http): New procedure, with code formerly in
'process-substitution'.
(process-substitution): Check for IPFS and call 'ipfs:restore-file-tree'
when IPFS is true.

guix/scripts/substitute.scm

@@ -42,6 +42,7 @@ (define-module (guix scripts substitute)
   #:use-module (guix progress)
   #:use-module ((guix build syscalls)
                 #:select (set-thread-name))
+  #:use-module ((guix ipfs) #:prefix ipfs:)
   #:use-module (ice-9 rdelim)
   #:use-module (ice-9 regex)
   #:use-module (ice-9 match)
@@ -281,7 +282,7 @@ (define (read-cache-info port)
 
 (define-record-type <narinfo>
   (%make-narinfo path uri uri-base compression file-hash file-size nar-hash nar-size
-                 references deriver system signature contents)
+                 references deriver system ipfs signature contents)
   narinfo?
   (path         narinfo-path)
   (uri          narinfo-uri)
@@ -294,6 +295,7 @@ (define-record-type <narinfo>
   (references   narinfo-references)
   (deriver      narinfo-deriver)
   (system       narinfo-system)
+  (ipfs         narinfo-ipfs)
   (signature    narinfo-signature)      ; canonical sexp
   ;; The original contents of a narinfo file.  This field is needed because we
   ;; want to preserve the exact textual representation for verification purposes.
@@ -335,7 +337,7 @@ (define (narinfo-maker str cache-url)
   "Return a narinfo constructor for narinfos originating from CACHE-URL.  STR
 must contain the original contents of a narinfo file."
   (lambda (path url compression file-hash file-size nar-hash nar-size
-                references deriver system signature)
+                references deriver system ipfs signature)
     "Return a new <narinfo> object."
     (%make-narinfo path
                    ;; Handle the case where URL is a relative URL.
@@ -352,6 +354,7 @@ (define (narinfo-maker str cache-url)
                      ((or #f "") #f)
                      (_ deriver))
                    system
+                   ipfs
                    (false-if-exception
                     (and=> signature narinfo-signature->canonical-sexp))
                    str)))
@@ -386,7 +389,7 @@ (define* (read-narinfo port #:optional url
                    (narinfo-maker str url)
                    '("StorePath" "URL" "Compression"
                      "FileHash" "FileSize" "NarHash" "NarSize"
-                     "References" "Deriver" "System"
+                     "References" "Deriver" "System" "IPFS"
                      "Signature"))))
 
 (define (narinfo-sha256 narinfo)
@@ -947,20 +950,8 @@ (define (valid? obj)
     (wtf
      (error "unknown `--query' command" wtf))))
 
-(define* (process-substitution store-item destination
+(define* (process-substitution/http narinfo destination uri
-                               #:key cache-urls acl print-build-trace?)
+                                    #:key print-build-trace?)
-  "Substitute STORE-ITEM (a store file name) from CACHE-URLS, and write it to
-DESTINATION as a nar file.  Verify the substitute against ACL."
-  (let* ((narinfo (lookup-narinfo cache-urls store-item
-                                  (cut valid-narinfo? <> acl)))
-         (uri     (and=> narinfo narinfo-uri)))
-    (unless uri
-      (leave (G_ "no valid substitute for '~a'~%")
-             store-item))
-
-    ;; Tell the daemon what the expected hash of the Nar itself is.
-    (format #t "~a~%" (narinfo-hash narinfo))
-
   (unless print-build-trace?
     (format (current-error-port)
             (G_ "Downloading ~a...~%") (uri->string uri)))
@@ -1001,7 +992,32 @@ (define* (process-substitution store-item destination
 
     ;; Skip a line after what 'progress-reporter/file' printed, and another
     ;; one to visually separate substitutions.
-      (display "\n\n" (current-error-port)))))
+    (display "\n\n" (current-error-port))))
+
+(define* (process-substitution store-item destination
+                               #:key cache-urls acl print-build-trace?)
+  "Substitute STORE-ITEM (a store file name) from CACHE-URLS, and write it to
+DESTINATION as a nar file.  Verify the substitute against ACL."
+  (let* ((narinfo (lookup-narinfo cache-urls store-item
+                                  (cut valid-narinfo? <> acl)))
+         (uri     (and=> narinfo narinfo-uri))
+         (ipfs    (and=> narinfo narinfo-ipfs)))
+    (unless uri
+      (leave (G_ "no valid substitute for '~a'~%")
+             store-item))
+
+    ;; Tell the daemon what the expected hash of the Nar itself is.
+    (format #t "~a~%" (narinfo-hash narinfo))
+
+    (if ipfs
+        (begin
+          (unless print-build-trace?
+            (format (current-error-port)
+                    (G_ "Downloading from IPFS ~s...~%") ipfs))
+          (ipfs:restore-file-tree ipfs destination))
+        (process-substitution/http narinfo destination uri
+                                   #:print-build-trace?
+                                   print-build-trace?))))
 
 
 ;;;