Process: if exec(3) fails with "Argument list too long", truncate the input

starting with the longest strings until it's small enough.
This way no string gets discarted completely in favor of a longer one before it.
Also, the computation happens only if necessary and in the already fork(2)ed child process.
The pragmatic implementation doesn't even try to mimic kernels' limits perfectly.

Author: Al2Klimov

lib/base/process.cpp

@@ -11,12 +11,15 @@
 #include "base/utility.hpp"
 #include "base/scriptglobal.hpp"
 #include "base/json.hpp"
+#include <algorithm>
+#include <cmath>
 #include <boost/algorithm/string/join.hpp>
 #include <boost/thread/once.hpp>
 #include <thread>
 #include <iostream>
 
 #ifndef _WIN32
+#	include <errno.h>
 #	include <execvpe.h>
 #	include <poll.h>
 #	include <string.h>
@@ -174,16 +177,48 @@ static Value ProcessSpawnImpl(struct msghdr *msgh, const Dictionary::Ptr& reques
 		sigemptyset(&mask);
 		sigprocmask(SIG_SETMASK, &mask, nullptr);
 
-		if (icinga2_execvpe(argv[0], argv, envp) < 0) {
-			char errmsg[512];
-			strcpy(errmsg, "execvpe(");
-			strncat(errmsg, argv[0], sizeof(errmsg) - strlen(errmsg) - 1);
-			strncat(errmsg, ") failed", sizeof(errmsg) - strlen(errmsg) - 1);
-			errmsg[sizeof(errmsg) - 1] = '\0';
-			perror(errmsg);
-		}
+		for (;;) {
+			if (icinga2_execvpe(argv[0], argv, envp) < 0) {
+				if (errno == E2BIG) {
+					size_t longest = 0;
+
+					for (auto strings : {argv, envp}) {
+						for (auto s (strings); *s; ++s) {
+							longest = std::max(longest, strlen(*s));
+						}
+					}
+
+					if (longest) {
+						size_t cutoff = exp2(floor(log2(longest)));
+
+						while (cutoff >= longest) {
+							cutoff /= 2u;
+						}
+
+						for (auto strings : {argv, envp}) {
+							for (auto s (strings); *s; ++s) {
+								auto str (*s);
+
+								if (strlen(str) > cutoff) {
+									str[cutoff] = 0;
+								}
+							}
+						}
 
-		_exit(128);
+						continue;
+					}
+				}
+
+				char errmsg[512];
+				strcpy(errmsg, "execvpe(");
+				strncat(errmsg, argv[0], sizeof(errmsg) - strlen(errmsg) - 1);
+				strncat(errmsg, ") failed", sizeof(errmsg) - strlen(errmsg) - 1);
+				errmsg[sizeof(errmsg) - 1] = '\0';
+				perror(errmsg);
+			}
+
+			_exit(128);
+		}
 	}
 
 	(void)close(fds[0]);