system: Add support for boot-time mapped devices.

* gnu/build/linux-boot.scm (boot-system): Add #:pre-mount parameter and
  honor it.
* gnu/system/linux-initrd.scm (base-initrd): Add #:mapped-devices
  parameter.  Add 'device-mapping-commands' variable, and use it to
  build the #:pre-mount argument of 'boot-system'.
* gnu/system.scm (mapped-device-user,
  operating-system-user-mapped-devices,
  operating-system-boot-mapped-devices): New procedures.
  (device-mapping-services): Use 'operating-system-user-mapped-devices'
  instead of 'operating-system-mapped-devices'.
  (operating-system-initrd-file): Call the initrd with #:mapped-devices.

gnu/build/linux-boot.scm

@@ -340,13 +340,14 @@ bailing out.~%root contents: ~s~%" (scandir "/"))
                       (linux-modules '())
                       qemu-guest-networking?
                       volatile-root?
+                      pre-mount
                       (mounts '()))
   "This procedure is meant to be called from an initrd.  Boot a system by
 first loading LINUX-MODULES (a list of absolute file names of '.ko' files),
 then setting up QEMU guest networking if QEMU-GUEST-NETWORKING? is true,
-mounting the file systems specified in MOUNTS, and finally booting into the
+calling PRE-MOUNT, mounting the file systems specified in MOUNTS, and finally
-new root if any.  The initrd supports kernel command-line options '--load',
+booting into the new root if any.  The initrd supports kernel command-line
-'--root', and '--repl'.
+options '--load', '--root', and '--repl'.
 
 Mount the root file system, specified by the '--root' command-line argument,
 if any.
@@ -403,6 +404,11 @@ to it are lost."
          (mkdir "/root/dev")
          (make-essential-device-nodes #:root "/root"))
 
+       (when (procedure? pre-mount)
+         ;; Do whatever actions are needed before mounting--e.g., installing
+         ;; device mappings.
+         (pre-mount))
+
        ;; Mount the specified file systems.
        (for-each mount-file-system
                  (remove root-mount-point? mounts))

gnu/system.scm

@@ -216,6 +216,34 @@ as 'needed-for-boot'."
                                            #:flags flags))))
                  file-systems)))
 
+(define (mapped-device-user device file-systems)
+  "Return a file system among FILE-SYSTEMS that uses DEVICE, or #f."
+  (let ((target (string-append "/dev/mapper/" (mapped-device-target device))))
+    (find (lambda (fs)
+            (string=? (file-system-device fs) target))
+          file-systems)))
+
+(define (operating-system-user-mapped-devices os)
+  "Return the subset of mapped devices that can be installed in
+user-land--i.e., those not needed during boot."
+  (let ((devices      (operating-system-mapped-devices os))
+        (file-systems (operating-system-file-systems os)))
+   (filter (lambda (md)
+             (let ((user (mapped-device-user md file-systems)))
+               (or (not user)
+                   (not (file-system-needed-for-boot? user)))))
+           devices)))
+
+(define (operating-system-boot-mapped-devices os)
+  "Return the subset of mapped devices that must be installed during boot,
+from the initrd."
+  (let ((devices      (operating-system-mapped-devices os))
+        (file-systems (operating-system-file-systems os)))
+   (filter (lambda (md)
+             (let ((user (mapped-device-user md file-systems)))
+               (and user (file-system-needed-for-boot? user))))
+           devices)))
+
 (define (device-mapping-services os)
   "Return the list of device-mapping services for OS as a monadic list."
   (sequence %store-monad
@@ -228,7 +256,7 @@ as 'needed-for-boot'."
                      (device-mapping-service target
                                              (open source target)
                                              (close source target))))
-                 (operating-system-mapped-devices os))))
+                 (operating-system-user-mapped-devices os))))
 
 (define (swap-services os)
   "Return the list of swap services for OS as a monadic list."
@@ -561,10 +589,14 @@ we're running in the final root."
               boot?))
             (operating-system-file-systems os)))
 
-  ;; TODO: Pass the mapped devices required by boot-time file systems to the
+  (define mapped-devices
-  ;; initrd.
+    (operating-system-boot-mapped-devices os))
-  (mlet %store-monad
+
-      ((initrd ((operating-system-initrd os) boot-file-systems)))
+  (define make-initrd
+    (operating-system-initrd os))
+
+  (mlet %store-monad ((initrd (make-initrd boot-file-systems
+                                           #:mapped-devices mapped-devices)))
     (return #~(string-append #$initrd "/initrd"))))
 
 (define (kernel->grub-label kernel)

gnu/system/linux-initrd.scm

@@ -126,14 +126,16 @@ initrd code."
 
 (define* (base-initrd file-systems
                       #:key
+                      (mapped-devices '())
                       qemu-networking?
                       virtio?
                       volatile-root?
                       (extra-modules '()))
-  ;; TODO: Support boot-time device mappings.
   "Return a monadic derivation that builds a generic initrd.  FILE-SYSTEMS is
 a list of file-systems to be mounted by the initrd, possibly in addition to
 the root file system specified on the kernel command line via '--root'.
+MAPPED-DEVICES is a list of device mappings to realize before FILE-SYSTEMS are
+mounted.
 
 When QEMU-NETWORKING? is true, set up networking with the standard QEMU
 parameters.  When VIRTIO? is true, load additional modules so the initrd can
@@ -191,6 +193,16 @@ loaded at boot time in the order in which they appear."
             (list unionfs-fuse/static)
             '())))
 
+  (define device-mapping-commands
+    ;; List of gexps to open the mapped devices.
+    (map (lambda (md)
+           (let* ((source (mapped-device-source md))
+                  (target (mapped-device-target md))
+                  (type   (mapped-device-type md))
+                  (open   (mapped-device-kind-open type)))
+             (open source target)))
+         mapped-devices))
+
   (mlet %store-monad ((kodir (flat-linux-module-directory linux-libre
                                                           linux-modules)))
     (expression->initrd
@@ -205,6 +217,8 @@ loaded at boot time in the order in which they appear."
                                             '#$helper-packages)))
 
          (boot-system #:mounts '#$(map file-system->spec file-systems)
+                      #:pre-mount (lambda ()
+                                    (and #$@device-mapping-commands))
                       #:linux-modules (map (lambda (file)
                                              (string-append #$kodir "/" file))
                                            '#$linux-modules)