Some notes regarding good error handling

Bug #209, among others.

src/error_handling/README.md

@@ -0,0 +1,19 @@
+# Comprehensively handling errors from the OS is a difficult job
+# In test, they rarely happen. In production, doing sane things
+# IN ALL CASES, is sane. 
+
+# Deciding what the sane things are, for each error return, is hard.
+
+# The example pinned.c file is a good example - What goes wrong if
+# ANY of these operations fail? What are the symptoms? Merely
+# removing the file does not mean that the daemon holding it open
+# notices
+
+See:
+
+https://github.com/LibreQoE/LibreQoS/issues/209
+https://github.com/LibreQoE/LibreQoS/issues/208
+https://github.com/LibreQoE/LibreQoS/issues/118   
+
+And several closed bugs, where EEXIST was not an error. Etc.
+

src/error_handling/pinned.c

@@ -0,0 +1,54 @@
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <errno.h>
+
+// Merely removing a file does not mean that it is no longer
+// in use. It has simply vanished from the filesystem
+// Also trapping on errors and reporting them is helpful
+
+static const char *pinned_maps[] = {
+"/sys/fs/bpf/map_traffic",
+"/sys/fs/bpf/map_ip_to_cpu_and_tc",
+"/sys/fs/bpf/cpu_map",
+"/sys/fs/bpf/cpus_available",
+"/sys/fs/bpf/packet_ts",
+"/sys/fs/bpf/flow_state",
+"/sys/fs/bpf/rtt_tracker",
+"/sys/fs/bpf/map_ip_to_cpu_and_tc_recip",
+"/sys/fs/bpf/tc/globals/map_txq_config",
+"/sys/fs/bpf/bifrost_interface_map",
+"/sys/fs/bpf/bifrost_vlan_map",
+NULL
+};
+
+int remove_pinned() {
+	const char **p = pinned_maps;
+	int err=0;
+	for (; *p != 0; p++) {
+		printf("Deleting file %s\n", *p);
+		if(unlink(*p) != 0 ) {
+			switch(errno) {
+			case EACCES:
+			case EBUSY:
+			case EFAULT:
+			case EIO:
+			case EISDIR:
+			case ELOOP:
+			case ENAMETOOLONG:
+			case ENOMEM:
+			case ENOTDIR:
+			case EPERM:
+			// Specific to unlinkat
+			case EBADF:
+			case EINVAL: // invaldd flags
+			default: perror("bad operation"); err++;
+			}
+		}
+	}
+	return err;
+}
+
+int main(int argc, char **argv) {
+		exit(remove_pinned());
+}