Move random number device check to runtime (fixes #1022)

Having it at compile time breaks cross-compilation and isn't really
right anyway.

ChangeLog

@@ -1,3 +1,10 @@
+2023-09-03  Jay Berkenbilt  <ejb@ql.org>
+
+        * Move check for random number device to runtime instead of
+        compile time. Since, by default, the crypto provider provides
+        random numbers, runtime determinination of a random number device
+        is usually not needed. Fixes #1022.
+
 2023-09-02  Jay Berkenbilt  <ejb@ql.org>
 
         * Bug fix from M. Holger: allow fix-qdf to read from pipe. Fixes #1010.

libqpdf/CMakeLists.txt

@@ -320,8 +320,6 @@ check_symbol_exists(fseeko "stdio.h" HAVE_FSEEKO)
 check_symbol_exists(fseeko64 "stdio.h" HAVE_FSEEKO64)
 check_symbol_exists(localtime_r "time.h" HAVE_LOCALTIME_R)
 check_symbol_exists(random "stdlib.h" HAVE_RANDOM)
-find_file(RANDOM_DEVICE
-  "urandom" "arandom" "arandom" PATHS "/dev" NO_DEFAULT_PATH)
 
 check_c_source_compiles(
 "#include <time.h>

libqpdf/SecureRandomDataProvider.cc

@@ -87,24 +87,27 @@ SecureRandomDataProvider::provideRandomData(unsigned char* data, size_t len)
         throw std::runtime_error("unable to generate secure random data");
     }
 
-# elif defined(RANDOM_DEVICE)
+# else
+    static std::unique_ptr<QUtil::FileCloser> random_device = []() {
+        FILE* f = fopen("/dev/urandom", "rb");
+        if (f == nullptr) {
+            f = fopen("/dev/arandom", "rb");
+        }
+        if (f == nullptr) {
+            f = fopen("/dev/random", "rb");
+        }
+        if (f == nullptr) {
+            throw std::runtime_error("unable to find device in /dev for generating random numbers");
+        }
+        return std::make_unique<QUtil::FileCloser>(f);
+    }();
 
-    // Optimization: wrap the file open and close in a class so that the file is closed in a
-    // destructor, then make this static to keep the file handle open.  Only do this if it can be
-    // done in a thread-safe fashion.
-    FILE* f = QUtil::safe_fopen(RANDOM_DEVICE, "rb");
-    size_t fr = fread(data, 1, len, f);
-    fclose(f);
+    size_t fr = fread(data, 1, len, random_device->f);
     if (fr != len) {
         throw std::runtime_error(
-            "unable to read " + std::to_string(len) + " bytes from " + std::string(RANDOM_DEVICE));
+            "unable to read " + std::to_string(len) + " bytes from random number device");
     }
 
-# else
-
-#  error \
-      "Don't know how to generate secure random numbers on this platform.  See random number generation in the top-level README.md"
-
 # endif
 }
 

libqpdf/qpdf/qpdf-config.h.in

@@ -24,8 +24,5 @@
 #cmakedefine HAVE_MALLOC_INFO 1
 #cmakedefine HAVE_OPEN_MEMSTREAM 1
 
-/* system random device (e.g. /dev/random) if any */
-#cmakedefine RANDOM_DEVICE "${RANDOM_DEVICE}"
-
 /* bytes in the size_t type */
 #cmakedefine SIZEOF_SIZE_T ${SIZEOF_SIZE_T}