solver: implement support for choosing default virtuals

By introducing a new package metadata field, `provider_priority`
(index letter `k`), we can specify default packages to satisfy a
virtual.

If a user wishes to select an alternative provider for the virtual,
a changeset swapping the default provider for the selected provider
will be generated by the dependency resolver.

src/apk_package.h

@@ -117,6 +117,7 @@ struct apk_package {
 	struct apk_dependency_array *depends, *install_if, *provides;
 	size_t installed_size, size;
 	time_t build_time;
+	unsigned short provider_priority;
 	unsigned repos : APK_MAX_REPOS;
 	unsigned marked : 1;
 	unsigned uninstallable : 1;

src/apk_solver_data.h

@@ -54,6 +54,7 @@ struct apk_solver_package_state {
 	unsigned seen : 1;
 	unsigned pkg_available : 1;
 	unsigned pkg_selectable : 1;
+	unsigned pkg_selected : 1;
 	unsigned tag_ok : 1;
 	unsigned tag_preferred : 1;
 	unsigned dependencies_used : 1;

src/package.c

@@ -812,6 +812,9 @@ int apk_pkg_add_info(struct apk_database *db, struct apk_package *pkg,
 	case 'c':
 		pkg->commit = apk_blob_cstr(value);
 		break;
+	case 'k':
+		pkg->provider_priority = apk_blob_pull_uint(&value, 10);
+		break;
 	case 'F': case 'M': case 'R': case 'Z': case 'r': case 'q':
 	case 'a': case 's': case 'f':
 		/* installed db entries which are handled in database.c */
@@ -850,6 +853,7 @@ static int read_info_line(void *ctx, apk_blob_t line)
 		{ "maintainer",	'm' },
 		{ "builddate",	't' },
 		{ "commit",	'c' },
+		{ "provider_priority", 'k' },
 	};
 	struct read_info_ctx *ri = (struct read_info_ctx *) ctx;
 	apk_blob_t l, r;
@@ -1125,6 +1129,10 @@ int apk_pkg_write_index_entry(struct apk_package *info,
 		apk_blob_push_blob(&bbuf, APK_BLOB_STR("\nc:"));
 		apk_blob_push_blob(&bbuf, APK_BLOB_STR(info->commit));
 	}
+	if (info->provider_priority) {
+		apk_blob_push_blob(&bbuf, APK_BLOB_STR("\nk:"));
+		apk_blob_push_uint(&bbuf, info->provider_priority, 10);
+	}
 	apk_blob_push_blob(&bbuf, APK_BLOB_STR("\n"));
 
 	if (APK_BLOB_IS_NULL(bbuf)) {

src/solver.c

@@ -299,6 +299,10 @@ static void apply_constraint(struct apk_solver_state *ss, struct apk_package *pp
 
 		if (is_provided)
 			inherit_pinning_and_flags(ss, pkg0, ppkg);
+
+		/* if a world-dependency is non-virtual, then the provider is going to be selected */
+		if (ppkg == NULL && dep->name == pkg0->name && pkg0->ss.pkg_selectable)
+			pkg0->ss.pkg_selected = 1;
 	}
 }
 
@@ -587,6 +591,16 @@ static int compare_providers(struct apk_solver_state *ss,
 	if (r)
 		return r;
 
+	/* Prefer already selected package. */
+	r = pkgA->ss.pkg_selected - pkgB->ss.pkg_selected;
+	if (r)
+		return r;
+
+	/* Prefer highest declared provider priority. */
+	r = pkgA->provider_priority - pkgB->provider_priority;
+	if (r)
+		return r;
+
 	/* Prefer lowest available repository */
 	return ffs(pkgB->repos) - ffs(pkgA->repos);
 }
@@ -639,18 +653,14 @@ static void select_package(struct apk_solver_state *ss, struct apk_name *name)
 
 	if (name->ss.requirers || name->ss.has_iif) {
 		foreach_array_item(p, name->providers) {
-			dbg_printf("  consider "PKG_VER_FMT" iif_triggered=%d, tag_ok=%d, selectable=%d\n",
-				PKG_VER_PRINTF(p->pkg), p->pkg->ss.iif_triggered, p->pkg->ss.tag_ok, p->pkg->ss.pkg_selectable);
+			dbg_printf("  consider "PKG_VER_FMT" iif_triggered=%d, tag_ok=%d, selectable=%d, selected=%d, provider_priority=%d\n",
+				PKG_VER_PRINTF(p->pkg), p->pkg->ss.iif_triggered, p->pkg->ss.tag_ok, p->pkg->ss.pkg_selectable,
+				p->pkg->ss.pkg_selected, p->pkg->provider_priority);
 			/* Ensure valid pinning and install-if trigger */
 			if (name->ss.requirers == 0 &&
 			    (!p->pkg->ss.iif_triggered ||
 			     !p->pkg->ss.tag_ok))
 				continue;
-			/* Virtual packages cannot be autoselected */
-			if (p->version == &apk_null_blob &&
-			    p->pkg->name->auto_select_virtual == 0 &&
-			    p->pkg->name->ss.requirers == 0)
-				continue;
 			if (compare_providers(ss, p, &chosen) > 0)
 				chosen = *p;
 		}

test/provides-swap.installed

@@ -0,0 +1,6 @@
+C:Q1EyN5AdpAOBJWKMR89pp/C66o+OE=
+P:mailreadplus
+V:1
+S:1
+I:1
+

test/provides-swap.test

@@ -0,0 +1,9 @@
+@ARGS
+--test-repo provides.repo
+--test-instdb provides-swap.installed
+--test-world mail-reader
+add mymailreader
+@EXPECT
+(1/2) Purging mailreadplus (1)
+(2/2) Installing mymailreader (1)
+OK: 0 MiB in 1 packages

test/provides.repo

@@ -45,6 +45,7 @@ V:1
 S:1
 I:1
 p:mail-reader
+k:1
 
 C:Q1EyN5AdpAOBJWKMR89pp/C77FFFF=
 P:server-a

test/provides2.test

@@ -2,7 +2,5 @@
 --test-repo provides.repo
 add mail-reader
 @EXPECT
-ERROR: unsatisfiable constraints:
-  mail-reader (virtual):
-    provided by: mymailreader-1 mailreadplus
-    required by: world[mail-reader]
+(1/1) Installing mailreadplus (1)
+OK: 0 MiB in 0 packages

test/provides8.test

@@ -0,0 +1,6 @@
+@ARGS
+--test-repo provides.repo
+add mail-reader mailreadplus
+@EXPECT
+(1/1) Installing mailreadplus (1)
+OK: 0 MiB in 0 packages