ash: improve set -x to quote strings as necessary

Basen on the patch from Martijn Dekker <martijn@inlv.org>

function                                             old     new   delta
evalcommand                                         1161    1302    +141
maybe_single_quote                                     -      60     +60
getoptscmd                                           527     546     +19
readtoken1                                          2819    2823      +4
localcmd                                             366     364      -2
evaltreenr                                           495     479     -16
evaltree                                             495     479     -16
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 3/3 up/down: 224/-34)           Total: 190 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

3 files changed, 94 insertions, 11 deletions

diff --git a/shell/ash.c b/shell/ash.c
index b28731eb1..a461bb7df 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -1742,7 +1742,7 @@ number(const char *s)
 }
 
 /*
- * Produce a possibly single quoted string suitable as input to the shell.
+ * Produce a single quoted string suitable as input to the shell.
  * The return string is allocated on the stack.
  */
 static char *
@@ -1786,6 +1786,47 @@ single_quote(const char *s)
         return stackblock();
 }
 
+/*
+ * Produce a possibly single quoted string suitable as input to the shell.
+ * If 'conditional' is nonzero, quoting is only done if the string contains
+ * non-shellsafe characters, or is identical to a shell keyword (reserved
+ * word); if it is zero, quoting is always done.
+ * If quoting was done, the return string is allocated on the stack,
+ * otherwise a pointer to the original string is returned.
+ */
+static const char *
+maybe_single_quote(const char *s)
+{
+        const char *p = s;
+
+        while (*p) {
+                /* Assuming ACSII */
+                /* quote ctrl_chars space !"#$%&'()* */
+                if (*p < '+')
+                        goto need_quoting;
+                /* quote ;<=>? */
+                if (*p >= ';' && *p <= '?')
+                        goto need_quoting;
+                /* quote `[\ */
+                if (*p == '`')
+                        goto need_quoting;
+                if (*p == '[')
+                        goto need_quoting;
+                if (*p == '\\')
+                        goto need_quoting;
+                /* quote {|}~ DEL and high bytes */
+                if (*p > 'z')
+                        goto need_quoting;
+                /* Not quoting these: +,-./ 0-9 :@ A-Z ]^_ a-z */
+                /* TODO: maybe avoid quoting % */
+                p++;
+        }
+        return s;
+
+ need_quoting:
+        return single_quote(s);
+}
+
 
 /* ============ nextopt */
 
@@ -9700,18 +9741,36 @@ evalcommand(union node *cmd, int flags)
 
         /* Print the command if xflag is set. */
         if (xflag) {
-                int n;
-                const char *p = " %s" + 1;
+                const char *pfx = "";
+
+                fdprintf(preverrout_fd, "%s", expandstr(ps4val()));
 
-                fdprintf(preverrout_fd, p, expandstr(ps4val()));
                 sp = varlist.list;
-                for (n = 0; n < 2; n++) {
-                        while (sp) {
-                                fdprintf(preverrout_fd, p, sp->text);
-                                sp = sp->next;
-                                p = " %s";
-                        }
-                        sp = arglist.list;
+                while (sp) {
+                        char *varval = sp->text;
+                        char *eq = strchrnul(varval, '=');
+                        if (*eq)
+                                eq++;
+                        fdprintf(preverrout_fd, "%s%.*s%s",
+                                pfx,
+                                (int)(eq - varval), varval,
+                                maybe_single_quote(eq)
+                        );
+                        sp = sp->next;
+                        pfx = " ";
+                }
+
+                sp = arglist.list;
+                while (sp) {
+                        fdprintf(preverrout_fd, "%s%s",
+                                pfx,
+                                /* always quote if matches reserved word: */
+                                findkwd(sp->text)
+                                ? single_quote(sp->text)
+                                : maybe_single_quote(sp->text)
+                        );
+                        sp = sp->next;
+                        pfx = " ";
                 }
                 safe_write(preverrout_fd, "\n", 1);
         }
diff --git a/shell/ash_test/ash-quoting/mode_x.right b/shell/ash_test/ash-quoting/mode_x.right
new file mode 100644
index 000000000..c2dd3550c
--- /dev/null
+++ b/shell/ash_test/ash-quoting/mode_x.right
@@ -0,0 +1,10 @@
++ var1=val
++ var2='one two'
++ true '%s\n' one 'two '"'"'three' four
++ this=command
++ 'this=command'
+./mode_x.tests: line 1: this=command: not found
++ true
++ true
++ 'if' true
+./mode_x.tests: line 1: if: not found
diff --git a/shell/ash_test/ash-quoting/mode_x.tests b/shell/ash_test/ash-quoting/mode_x.tests
new file mode 100755
index 000000000..16dae3f4b
--- /dev/null
+++ b/shell/ash_test/ash-quoting/mode_x.tests
@@ -0,0 +1,14 @@
+set -x
+
+var1=val
+var2='one two'
+true %s\\n one "two 'three" four
+
+# assignment:
+this=command
+# NOT assignment, +x code should show it quoted:
+"this=command"
+
+if true; then true; fi
+# +x code should quote 'if' here:
+"if" true