solver, db: implement repository pinning

Improves /etc/apk/repositories format so you can say:
http://nl.alpinelinux.org/alpine/v2.3/main
@edge http://nl.alpinelinux.org/alpine/edge/main
@testing http://nl.alpinelinux.org/alpine/edge/testing

After which you can pin dependencies to these tags using:
 apk add stableapp newapp@edge bleedingapp@testing

Apk will now by default only use the untagged repositories,
but adding a tag to specific dependency:
 1. will prefer that tag for the name
 2. allowing pulling in dependencies from that tag (though,
    it prefers untagged packages to satisfy deps if possible)

fixes #575

1 files changed, 8 insertions, 4 deletions

diff --git a/src/blob.c b/src/blob.c
index aaaa377..8cf4076 100644
--- a/src/blob.c
+++ b/src/blob.c
@@ -44,8 +44,10 @@ int apk_blob_spn(apk_blob_t blob, const char *accept, apk_blob_t *l, apk_blob_t
 
 	for (i = 0; i < blob.len; i++) {
 		if (strchr(accept, blob.ptr[i]) == NULL) {
-			*l = APK_BLOB_PTR_LEN(blob.ptr, i);
-			*r = APK_BLOB_PTR_LEN(blob.ptr+i, blob.len-i);
+			if (l != NULL)
+				*l = APK_BLOB_PTR_LEN(blob.ptr, i);
+			if (r != NULL)
+				*r = APK_BLOB_PTR_LEN(blob.ptr+i, blob.len-i);
 			return 1;
 		}
 	}
@@ -58,8 +60,10 @@ int apk_blob_cspn(apk_blob_t blob, const char *reject, apk_blob_t *l, apk_blob_t
 
 	for (i = 0; i < blob.len; i++) {
 		if (strchr(reject, blob.ptr[i]) != NULL) {
-			*l = APK_BLOB_PTR_LEN(blob.ptr, i);
-			*r = APK_BLOB_PTR_LEN(blob.ptr+i, blob.len-i);
+			if (l != NULL)
+				*l = APK_BLOB_PTR_LEN(blob.ptr, i);
+			if (r != NULL)
+				*r = APK_BLOB_PTR_LEN(blob.ptr+i, blob.len-i);
 			return 1;
 		}
 	}