Ignore comment line contents in macro files (#1659)

Previously %{ and similar in macro file comment line would cause the
line continuation logic to trigger and silently eat macro definitions
up to the next empty line. Since 75275a87cf
we permit empty lines inside macro definitions, which would cause the
whole remaining file to be silently skipped (RhBug:1953910)

Only ever parse macro file lines starting with %, and add a test for
the case.

Actual patch by Michael Schroeder, testcase by undersigned.

Fixes: #1659

rpmio/macro.c

@@ -226,6 +226,14 @@ rdcl(char * buf, size_t size, FILE *f)
 	    nb--;
 	if (*q == 0)
 	    break;			/* no newline found, EOF */
+	if (p == buf) {
+            while (*p && isblank(*p))
+                p++;
+            if (*p != '%') {		/* only parse actual macro */
+                *q = '\0';		/* trim trailing \r, \n */
+                break;
+            }
+        }
 	for (; p < q; p++) {
 	    switch (*p) {
 		case '\\':

tests/data/macros.testfile

@@ -5,6 +5,13 @@
 }
 %second %{?def:macro_2}
 
+# empty lines inside a %{ block
+%empty0 %{expand:
+some
+
+thing
+}
+
 %comment1 %{expand:
 read
 %dnl comment

tests/rpmmacro.at

@@ -945,6 +945,23 @@ read
 [])
 AT_CLEANUP
 
+AT_SETUP([macro file empty lines])
+AT_KEYWORDS([macros])
+AT_CHECK([
+runroot rpm \
+	--macros /data/macros.testfile \
+	--eval "%{empty0}"
+],
+[0],
+[
+some
+
+thing
+
+],
+[])
+AT_CLEANUP
+
 AT_SETUP([macro traceback])
 AT_KEYWORDS([macros])
 AT_CHECK([