[bug#60802] [PATCH v4 1/2] platforms: Raise an exception when no suitable platform is found.

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

This was motivated by #60786, which produced a cryptic, hard to understand
backtrace.

Given the following reproducer:
    (use-modules (guix packages)
                 (gnu packages cross-base))

    (define linux-libre-headers-cross-mips64el-linux-gnuabi64
      (cross-kernel-headers "mips64el-linux-gnuabi64"))

    (package-arguments linux-libre-headers-cross-mips64el-linux-gnuabi64)

Before this change:
    ice-9/boot-9.scm:1685:16: In procedure raise-exception:
    In procedure struct-vtable: Wrong type argument in position 1 (expecting struct): #f

After this change:
    ice-9/boot-9.scm:1685:16: In procedure raise-exception:
    ERROR:
      1. &platform-not-found-error: "mips64el-linux-gnuabi64"

* guix/platform.scm (&platform-not-found-error): New condition.
(platform-not-found-error?): New predicate.
(false-if-platform-not-found): New syntax.
(lookup-platform-by-system): Raise an exception when no platform is found.
Update documentation.
(lookup-platform-by-target): Likewise.
(lookup-platform-by-target-or-system): Likewise, and guard lookup calls with
false-if-platform-not-found.
* gnu/packages/bootstrap.scm (glibc-dynamic-linker): Handle
lookup-platform-by-system call to preserve existing behavior.

---

Changes in v4:
- Use (srfi srfi-35) condition instead of (ice-9 exceptions)
- Add a target-or-system field to the condition type for extra information

Changes in v3:
- Use false-if-platform-not-found instead of false-if-exception in (gnu
packages bootstrap)
- Do not export make-platform-not-found-error constructor

Changes in v2:
- Add false-if-platform-not-found syntax
- Use it while evaluating lookup-platform-by-target-or-system

 gnu/packages/bootstrap.scm |  3 ++-
 guix/platform.scm          | 55 +++++++++++++++++++++++++++++---------
 2 files changed, 44 insertions(+), 14 deletions(-)

diff --git a/gnu/packages/bootstrap.scm b/gnu/packages/bootstrap.scm
index d2914fb5a7..9ea1a3e4d1 100644
--- a/gnu/packages/bootstrap.scm
+++ b/gnu/packages/bootstrap.scm
@@ -315,7 +315,8 @@ (define* (glibc-dynamic-linker
                                  (%current-system))))
   "Return the name of Glibc's dynamic linker for SYSTEM."
   ;; See the 'SYSDEP_KNOWN_INTERPRETER_NAMES' cpp macro in libc.
-  (let ((platform (lookup-platform-by-system system)))
+  (let ((platform (false-if-platform-not-found
+                   (lookup-platform-by-system system))))
     (cond
      ((platform? platform)
       (platform-glibc-dynamic-linker platform))
diff --git a/guix/platform.scm b/guix/platform.scm
index f873913fe0..a2d95ab507 100644
--- a/guix/platform.scm
+++ b/guix/platform.scm
@@ -22,6 +22,8 @@ (define-module (guix platform)
   #:use-module (guix records)
   #:use-module (guix ui)
   #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
   #:export (platform
             platform?
             platform-target
@@ -29,6 +31,10 @@ (define-module (guix platform)
             platform-linux-architecture
             platform-glibc-dynamic-linker
 
+            &platform-not-found-error
+            platform-not-found-error?
+            false-if-platform-not-found
+
             platform-modules
             platforms
             lookup-platform-by-system
@@ -70,6 +76,20 @@ (define-record-type* <platform> platform make-platform
                         (default #false))
   (glibc-dynamic-linker platform-glibc-dynamic-linker))
 
+\f
+;;;
+;;; Exceptions.
+;;;
+(define-condition-type &platform-not-found-error &error
+  platform-not-found-error?
+  (target-or-system platform-not-found-error-target-or-system))
+
+(define-syntax-rule (false-if-platform-not-found exp)
+  "Evaluate EXP but return #f if it raises a platform-not-found-error?
+exception."
+  (guard (ex ((platform-not-found-error? ex) #f))
+    exp))
+
 \f
 ;;;
 ;;; Platforms.
@@ -94,23 +114,32 @@ (define platforms
                                    (platform-modules)))))
 
 (define (lookup-platform-by-system system)
-  "Return the platform corresponding to the given SYSTEM."
-  (find (lambda (platform)
-          (let ((s (platform-system platform)))
-            (and (string? s) (string=? s system))))
-        (platforms)))
+  "Return the platform corresponding to the given SYSTEM.  Raise
+&PLATFORM-NOT-FOUND-ERROR when no platform could be found."
+  (or (find (lambda (platform)
+              (let ((s (platform-system platform)))
+                (and (string? s) (string=? s system))))
+            (platforms))
+      (raise-exception (condition (&platform-not-found-error
+                                   (target-or-system system))))))
 
 (define (lookup-platform-by-target target)
-  "Return the platform corresponding to the given TARGET."
-  (find (lambda (platform)
-          (let ((t (platform-target platform)))
-            (and (string? t) (string=? t target))))
-        (platforms)))
+  "Return the platform corresponding to the given TARGET.  Raise
+&PLATFORM-NOT-FOUND-ERROR when no platform could be found."
+  (or (find (lambda (platform)
+              (let ((t (platform-target platform)))
+                (and (string? t) (string=? t target))))
+            (platforms))
+      (raise-exception (condition (&platform-not-found-error
+                                   (target-or-system target))))))
 
 (define (lookup-platform-by-target-or-system target-or-system)
-  "Return the platform corresponding to the given TARGET or SYSTEM."
-  (or (lookup-platform-by-target target-or-system)
-      (lookup-platform-by-system target-or-system)))
+  "Return the platform corresponding to the given TARGET or SYSTEM.  Raise
+&PLATFORM-NOT-FOUND-ERROR when no platform could be found."
+  (or (false-if-platform-not-found (lookup-platform-by-target target-or-system))
+      (false-if-platform-not-found (lookup-platform-by-system target-or-system))
+      (raise-exception (condition (&platform-not-found-error
+                                   (target-or-system target-or-system))))))
 
 (define (platform-system->target system)
   "Return the target matching the given SYSTEM if it exists or false
-- 
2.38.1