Allow auto-detecting the default backend.

configure.ac

@@ -43,6 +43,20 @@ AC_ARG_WITH([odk-executable],
               [with_odk_executable="/tools/odk.py"])
 AC_DEFINE_UNQUOTED([ODK_EXECUTABLE], ["$with_odk_executable"], [Name or path of the odk script])
 
+dnl Default backend
+AC_ARG_WITH([default-backend],
+            [AS_HELP_STRING([--with-default-backend],
+                            [Name of the backend to use by default. One of docker, singularity, apple, native, or none.])],
+            [],
+            [with_default_backend=docker])
+AS_CASE([$with_default_backend],
+        [docker],      [AC_DEFINE([ODK_DEFAULT_BACKEND], [odk_backend_docker_init], [Docker backend])],
+        [singularity], [AC_DEFINE([ODK_DEFAULT_BACKEND], [odk_backend_singularity_init], [Singularity backend])],
+        [apple],       [AC_DEFINE([ODK_DEFAULT_BACKEND], [odk_backend_apple_init], [Apple Container backend])],
+        [native],      [AC_DEFINE([ODK_DEFAULT_BACKEND], [odk_backend_native_init], [Native backend])],
+        [none],        [AC_DEFINE([ODK_DEFAULT_BACKEND], [NULL], [No default backend])],
+        [AC_MSG_ERROR(["Unsupported backend: ${with_default_backend}"])])
+
 dnl Host-based tweaks
 AC_CANONICAL_HOST
 AS_CASE([$host],
@@ -67,5 +81,6 @@ Configuration complete
 
 Host:               '${host}'
 Compiler:           '${CC} ${CFLAGS} ${CPPFLAGS}'
+Default backend:    '${with_default_backend}'
 
 "

doc/odkrun.1.in

@@ -13,6 +13,7 @@ odkrun - ODK Runner tool
 .RB [ -t | --tag
 .IR tag ]
 .RB [ -l | --lite ]
+.RB [ -D | --docker ]
 .RB [ -s | --singulary ]
 .RB [ --apple ]
 .RB [ -n | --native ]
@@ -62,9 +63,14 @@ Use the \fIobolibrary/odklite\fR image.
 
 .SH BACKEND OPTIONS
 .TP
+.BR -D ", " --docker
+Run the container with Docker. This is normally the default
+backend, unless specified otherwise at configure time.
+.TP
 .BR -s ", " --singularity
 Run the container with Singularity rather than Docker. This
 is experimental but should mostly work.
+.TP
 .BR --apple
 Run the container with Apple Container rather than Docker.
 This is only available on macOS. This is experimental but

src/backend-apple.c

@@ -38,7 +38,6 @@
 #include <string.h>
 #include <errno.h>
 
-#include "procutil.h"
 #include "util.h"
 #include "backend-docker.h"
 

src/backend-apple.h

@@ -40,6 +40,12 @@ extern "C" {
 int
 odk_backend_apple_init(odk_backend_t *);
 
+#if defined(ODK_RUNNER_MACOS)
+#define odk_backend_apple_available() check_process("container", "--version")
+#else
+#define odk_backend_apple_available() 0
+#endif
+
 #ifdef __cpluscplus
 }
 #endif

src/backend-docker.c

@@ -42,7 +42,6 @@
 #include <unistd.h> /* for getuid/getgid */
 #endif
 
-#include "procutil.h"
 #include "util.h"
 
 #define DOCKER_SSH_SOCKET "/run/host-services/ssh-auth.sock"

src/backend-docker.h

@@ -34,6 +34,7 @@
 #include <memreg.h>
 
 #include "backend.h"
+#include "procutil.h"
 
 #ifdef __cpluscplus
 extern "C" {
@@ -45,6 +46,8 @@ odk_backend_docker_build_command(mem_registry_t *, odk_run_config_t *, char **);
 int
 odk_backend_docker_init(odk_backend_t *);
 
+#define odk_backend_docker_available()  check_process("docker", "--version")
+
 #ifdef __cpluscplus
 }
 #endif

src/backend-native.h

@@ -40,6 +40,12 @@ extern "C" {
 int
 odk_backend_native_init(odk_backend_t *);
 
+#if !defined(ODK_RUNNER_WINDOWS)
+#define odk_backend_native_available() 1
+#else
+#define odk_backend_native_available() 0
+#endif
+
 #ifdef __cpluscplus
 }
 #endif

src/backend-singularity.c

@@ -43,7 +43,6 @@
 #include <memreg.h>
 #include <sbuffer.h>
 
-#include "procutil.h"
 #include "util.h"
 
 #define SINGULARITY_SSH_SOCKET "/run/host-services/ssh-auth.sock"

src/backend-singularity.h

@@ -32,6 +32,7 @@
 #define ICP20240622_BACKEND_SINGULARITY_H
 
 #include "backend.h"
+#include "procutil.h"
 
 #ifdef __cpluscplus
 extern "C" {
@@ -40,6 +41,8 @@ extern "C" {
 int
 odk_backend_singularity_init(odk_backend_t *);
 
+#define odk_backend_singularity_available() check_process("singularity", "--version")
+
 #ifdef __cpluscplus
 }
 #endif

src/odkrun.c

@@ -82,6 +82,8 @@ Start a ODK container.\n");
 ");
 
     puts("Backend options:\n\
+    -D, --docker        Run the container with Docker. This is normally\n\
+                        the default.\n\
     -s, --singulary     Run the container with Singularity rather\n\
                         than Docker (experimental).");
 #if defined(ODK_RUNNER_MACOS)
@@ -449,7 +451,7 @@ main(int argc, char **argv)
     char *opt_value, *java_mem = NULL;
     odk_run_config_t cfg;
     odk_backend_t backend = { 0 };
-    odk_backend_init backend_init = odk_backend_docker_init;
+    odk_backend_init backend_init = ODK_DEFAULT_BACKEND;
 
     struct option options[] = {
         { "help",           0, NULL, 'h' },
@@ -458,6 +460,7 @@ main(int argc, char **argv)
         { "image",          1, NULL, 'i' },
         { "tag",            1, NULL, 't' },
         { "lite",           0, NULL, 'l' },
+        { "docker",         0, NULL, 'D' },
         { "singularity",    0, NULL, 's' },
 #if defined (ODK_RUNNER_MACOS)
         { "apple",          0, NULL, 'a' },
@@ -481,7 +484,7 @@ main(int argc, char **argv)
 
     odk_init_config(&cfg);
 
-    while ( (c = getopt_long(argc, argv, "+hvdi:t:lse:k:Km:" BACKEND_OPTS,
+    while ( (c = getopt_long(argc, argv, "+hvdi:t:lse:k:Km:D" BACKEND_OPTS,
                              options, NULL)) != -1 ) {
         switch ( c ) {
         case 'h':
@@ -513,6 +516,10 @@ main(int argc, char **argv)
             odk_set_image_name(&cfg, "obolibrary/odklite", 0);
             break;
 
+        case 'D':
+            backend_init = odk_backend_docker_init;
+            break;
+
         case 's':
             backend_init = odk_backend_singularity_init;
             break;
@@ -585,6 +592,19 @@ main(int argc, char **argv)
 #endif
     }
 
+    if ( ! backend_init ) {
+        if ( odk_backend_docker_available() )
+            backend_init = odk_backend_docker_init;
+        else if ( odk_backend_singularity_available() )
+            backend_init = odk_backend_singularity_init;
+        else if ( odk_backend_apple_available() )
+            backend_init = odk_backend_apple_init;
+        else if ( odk_backend_native_available() )
+            backend_init = odk_backend_native_init;
+        else
+            errx(EXIT_FAILURE, "No backend specified");
+    }
+
     if ( backend_init(&backend) == -1 )
         err(EXIT_FAILURE, "Cannot initialise backend");
 

src/procutil.c

@@ -116,3 +116,26 @@ spawn_process(char **argv)
 #endif
     return -1;
 }
+
+/**
+ * Spawns a process and checks that it exits with a non-error status.
+ * This is primarily intended to check whether a given command is
+ * available on the system's PATH.
+ *
+ * @param command  The command to run.
+ * @param argument A single argument for the command. May be NULL.
+ *
+ * @return A non-zero value if the process executed and returned
+ *         normally, otherwise zero.
+ */
+int
+check_process(const char *command, const char *argument)
+{
+    char *argv[3];
+
+    argv[0] = (char *)command;
+    argv[1] = (char *)argument;
+    argv[2] = NULL;
+
+    return spawn_process(argv) == 0;
+}

src/procutil.h

@@ -38,6 +38,9 @@ extern "C" {
 int
 spawn_process(char **);
 
+int
+check_process(const char *, const char *);
+
 #ifdef __cplusplus
 }
 #endif