Add support for breaking into debugger for Linux

Use Linux-specific /proc/$PID/status file to check whether we're being
debugged and a generic raise(SIGTRAP) to actually break into the debugger.

include/internal/catch_debugger.h

@@ -35,6 +35,10 @@ namespace Catch{
         #endif
     #endif
 
+#elif defined(CATCH_PLATFORM_LINUX)
+    #include <signal.h>
+
+    #define CATCH_BREAK_INTO_DEBUGGER() if( Catch::isDebuggerActive() ) { raise(SIGTRAP); }
 #elif defined(_MSC_VER)
     #define CATCH_BREAK_INTO_DEBUGGER() if( Catch::isDebuggerActive() ) { __debugbreak(); }
 #elif defined(__MINGW32__)

include/internal/catch_debugger.hpp

@@ -61,6 +61,33 @@
         }
     } // namespace Catch
 
+#elif defined(CATCH_PLATFORM_LINUX)
+    #include <fstream>
+    #include <string>
+
+    namespace Catch{
+        // The standard POSIX way of detecting a debugger is to attempt to
+        // ptrace() the process, but this needs to be done from a child and not
+        // this process itself to still allow attaching to this process later
+        // if wanted, so is rather heavy. Under Linux we have the PID of the
+        // "debugger" (which doesn't need to be gdb, of course, it could also
+        // be strace, for example) in /proc/$PID/status, so just get it from
+        // there instead.
+        bool isDebuggerActive(){
+            std::ifstream in("/proc/self/status");
+            for( std::string line; std::getline(in, line); ) {
+                static const int PREFIX_LEN = 11;
+                if( line.compare(0, PREFIX_LEN, "TracerPid:\t") == 0 ) {
+                    // We're traced if the PID is not 0 and no other PID starts
+                    // with 0 digit, so it's enough to check for just a single
+                    // character.
+                    return line.length() > PREFIX_LEN && line[PREFIX_LEN] != '0';
+                }
+            }
+
+            return false;
+        }
+    } // namespace Catch
 #elif defined(_MSC_VER)
     extern "C" __declspec(dllimport) int __stdcall IsDebuggerPresent();
     namespace Catch {

include/internal/catch_platform.h

@@ -15,6 +15,8 @@
 #define CATCH_PLATFORM_IPHONE
 #elif defined(WIN32) || defined(__WIN32__) || defined(_WIN32) || defined(_MSC_VER)
 #define CATCH_PLATFORM_WINDOWS
+#elif defined(linux) || defined(__linux) || defined(__linux__)
+#define CATCH_PLATFORM_LINUX
 #endif
 
 #endif // TWOBLUECUBES_CATCH_PLATFORM_H_INCLUDED