make: allow empty commands

pdpmake didn't allow rules to have empty commands. There are
circumstances where this may be useful.

Make the following changes:

- Add a flag to readline() to indicate the next line is expected
  to be a command.  If this flag is true and the input line starts
  with a tab return it immediately, thus skipping the check for
  an empty line or comment line.

- In docmds() skip tabs and spaces after a command prefix.  If the
  resulting command is empty don't print it or try to execute it.

- In newcmd() allow empty commands.

Adds 48-96 bytes.

(pdpmake GitHub issue 56)

2 files changed, 23 insertions, 10 deletions

diff --git a/miscutils/make.c b/miscutils/make.c
index c65d32e78..1dfbd8c44 100644
--- a/miscutils/make.c
+++ b/miscutils/make.c
@@ -456,9 +456,6 @@ newcmd(struct cmd **cphead, char *str)
         while (isspace(*str))
                 str++;
 
-        if (*str == '\0')               // No command, leave current head unchanged
-                return;
-
         while (*cphead)
                 cphead = &(*cphead)->c_next;
         *cphead = xzalloc(sizeof(struct cmd));
@@ -1708,7 +1705,7 @@ make_fgets(char *s, int size, FILE *fd)
  * Ignore comment lines.  Return NULL on EOF.
  */
 static char *
-readline(FILE *fd)
+readline(FILE *fd, int want_command)
 {
         char *p, *str = NULL;
         int pos = 0;
@@ -1748,6 +1745,9 @@ readline(FILE *fd)
                 }
                 dispno = lineno;
 
+                if (want_command && *str == '\t')
+                        return str;
+
                 // Check for comment lines and lines that are conditionally skipped.
                 p = str;
                 while (isblank(*p))
@@ -2043,7 +2043,7 @@ input(FILE *fd, int ilevel)
         bool minus;
 
         lineno = 0;
-        str1 = readline(fd);
+        str1 = readline(fd, FALSE);
         while (str1) {
                 str2 = NULL;
                 if (*str1 == '\t')      // Command without target
@@ -2292,7 +2292,7 @@ input(FILE *fd, int ilevel)
 
                 // Create list of commands
                 startno = dispno;
-                while ((str2 = readline(fd)) && *str2 == '\t') {
+                while ((str2 = readline(fd, TRUE)) && *str2 == '\t') {
                         newcmd(&cp, process_command(str2));
                         free(str2);
                 }
@@ -2343,7 +2343,7 @@ input(FILE *fd, int ilevel)
  end_loop:
                 free(str1);
                 dispno = lineno;
-                str1 = str2 ? str2 : readline(fd);
+                str1 = str2 ? str2 : readline(fd, FALSE);
                 free(copy);
                 free(expanded);
 #if ENABLE_FEATURE_MAKE_POSIX
@@ -2427,7 +2427,9 @@ docmds(struct name *np, struct cmd *cp)
                                 sdomake = TRUE + 1;
                         else
                                 break;
-                        q++;
+                        do {
+                                q++;
+                        } while (isblank(*q));
                 }
 
                 if (sdomake > TRUE) {
@@ -2437,7 +2439,7 @@ docmds(struct name *np, struct cmd *cp)
                 } else if (!sdomake)
                         ssilent = dotouch;
 
-                if (!ssilent) {
+                if (!ssilent && *q != '\0') {   // Ignore empty commands
                         puts(q);
                         fflush_all();
                 }
@@ -2448,7 +2450,7 @@ docmds(struct name *np, struct cmd *cp)
                         continue;
                 }
 
-                if (sdomake) {
+                if (sdomake && *q != '\0') {    // Ignore empty commands
                         // Get the shell to execute it
                         int status;
                         char *cmd = !signore && posix ? auto_concat("set -e;", q) : q;
diff --git a/testsuite/make.tests b/testsuite/make.tests
index 2425ae99d..5485233f6 100755
--- a/testsuite/make.tests
+++ b/testsuite/make.tests
@@ -31,6 +31,17 @@ testing "make .DEFAULT rule for prerequisite" \
 target: source
 '
 
+mkdir make.tempdir && cd make.tempdir || exit 1
+touch target.xyz
+testing "make empty command overrides inference rule" \
+        "make -f - target 2>/dev/null" "" "" '
+.SUFFIXES: .xyz
+.xyz:
+        @echo xyz
+target: ;
+'
+cd .. || exit 1; rm -rf make.tempdir 2>/dev/null
+
 # Macros should be expanded before suffix substitution.  The suffixes
 # can be obtained by macro expansion.
 testing "make macro expansion and suffix substitution" \