Linux: change how kernel threads are detected

Use the same method that ps and top use to determine if a
process is a kernel thread on Linux: check if cmdline is empty.

Thanks to @wangqr's investigation reported here:
https://github.com/hishamhm/htop/issues/761#issuecomment-375306069

Fixes #761.

linux/LinuxProcess.c

@@ -93,6 +93,7 @@ typedef enum LinuxProcessFields {
 
 typedef struct LinuxProcess_ {
    Process super;
+   bool isKernelThread;
    IOPriority ioPriority;
    unsigned long int cminflt;
    unsigned long int cmajflt;
@@ -142,7 +143,7 @@ typedef struct LinuxProcess_ {
 } LinuxProcess;
 
 #ifndef Process_isKernelThread
-#define Process_isKernelThread(_process) (_process->pgrp == 0)
+#define Process_isKernelThread(_process) ((LinuxProcess*)(_process)->isKernelThread)
 #endif
 
 #ifndef Process_isUserlandThread

linux/LinuxProcess.h

@@ -85,6 +85,7 @@ typedef enum LinuxProcessFields {
 
 typedef struct LinuxProcess_ {
    Process super;
+   bool isKernelThread;
    IOPriority ioPriority;
    unsigned long int cminflt;
    unsigned long int cmajflt;
@@ -134,7 +135,7 @@ typedef struct LinuxProcess_ {
 } LinuxProcess;
 
 #ifndef Process_isKernelThread
-#define Process_isKernelThread(_process) (_process->pgrp == 0)
+#define Process_isKernelThread(_process) (((LinuxProcess*)(_process))->isKernelThread)
 #endif
 
 #ifndef Process_isUserlandThread

linux/LinuxProcessList.c

@@ -677,9 +677,6 @@ static void setCommand(Process* process, const char* command, int len) {
 }
 
 static bool LinuxProcessList_readCmdlineFile(Process* process, const char* dirname, const char* name) {
-   if (Process_isKernelThread(process))
-      return true;
-
    char filename[MAX_NAME+1];
    xSnprintf(filename, MAX_NAME, "%s/%s/cmdline", dirname, name);
    int fd = open(filename, O_RDONLY);
@@ -691,7 +688,10 @@ static bool LinuxProcessList_readCmdlineFile(Process* process, const char* dirna
    close(fd);
    int tokenEnd = 0; 
    int lastChar = 0;
-   if (amtRead <= 0) {
+   if (amtRead == 0) {
+      ((LinuxProcess*)process)->isKernelThread = true;
+      return true;
+   } else if (amtRead < 0) {
       return false;
    }
    for (int i = 0; i < amtRead; i++) {