Linux: make Process::getPath (and dfhack.getDFPath()) not depend on cwd, for consistency with other platforms

docs/changelog.txt

@@ -39,6 +39,7 @@ changelog.txt uses a syntax similar to RST, with a few special sequences:
 ## Fixes
 - Fixed a segfault when attempting to start a headless session with a graphical PRINT_MODE setting
 - Fixed an issue with the macOS launcher failing to un-quarantine some files
+- Linux: fixed ``dfhack.getDFPath()`` (Lua) and ``Process::getPath()`` (C++) to always return the DF root path, even if the working directory has changed
 - `labormanager`: fixed handling of new jobs in 0.47
 - `embark-assistant`: fixed a couple of incursion handling bugs.
 - Fixed ``Units::isEggLayer``, ``Units::isGrazer``, ``Units::isMilkable``, ``Units::isTrainableHunting``, ``Units::isTrainableWar``, and ``Units::isTamable`` ignoring the unit's caste

library/Process-linux.cpp

@@ -23,11 +23,12 @@ distribution.
 */
 
 #include "Internal.h"
+
 #include <dirent.h>
 #include <errno.h>
-#include <unistd.h>
 #include <sys/mman.h>
 #include <sys/time.h>
+#include <unistd.h>
 
 #include <string>
 #include <vector>
@@ -180,12 +181,28 @@ uint32_t Process::getTickCount()
 
 string Process::getPath()
 {
-    const char * cwd_name = "/proc/self/cwd";
-    char target_name[1024];
-    int target_result;
-    target_result = readlink(cwd_name, target_name, sizeof(target_name));
-    target_name[target_result] = '\0';
-    return(string(target_name));
+    static string cached_path;
+    if (cached_path.empty())
+    {
+        const char *exe_name = "/proc/self/exe";
+        char exe_path[1024];
+        int length = readlink(exe_name, exe_path, sizeof(exe_path));
+        if (length > 0)
+        {
+            exe_path[length] = '\0';
+            string path_string = exe_path;
+            // DF lives in libs, so move up a folder
+            cached_path = path_string.substr(0, path_string.find_last_of("/", path_string.find_last_of("/") - 1));
+        }
+        else
+        {
+            perror("readlink(/proc/self/exe) failed");
+            fprintf(stderr, "  length=%i\n", length);
+            cached_path = ".";
+        }
+        fprintf(stderr, "Resolved DF root to %s\n", cached_path.c_str());
+    }
+    return cached_path;
 }
 
 int Process::getPID()

test/core.lua

@@ -0,0 +1,34 @@
+local function clean_path(p)
+    -- todo: replace with dfhack.filesystem call?
+    return p:gsub('\\', '/'):gsub('//', '/'):gsub('/$', '')
+end
+
+local fs = dfhack.filesystem
+local old_cwd = clean_path(fs.getcwd())
+local function restore_cwd()
+    fs.chdir(old_cwd)
+end
+
+function test.getDFPath()
+    expect.eq(clean_path(dfhack.getDFPath()), old_cwd)
+end
+
+function test.getDFPath_chdir()
+    dfhack.with_finalize(restore_cwd, function()
+        fs.chdir('data')
+        expect.eq(clean_path(dfhack.getDFPath()), old_cwd)
+        expect.ne(clean_path(dfhack.getDFPath()), clean_path(fs.getcwd()))
+    end)
+end
+
+function test.getHackPath()
+    expect.eq(clean_path(dfhack.getHackPath()), clean_path(dfhack.getDFPath() .. '/hack/'))
+end
+
+function test.getHackPath_chdir()
+    dfhack.with_finalize(restore_cwd, function()
+        fs.chdir('hack')
+        expect.eq(clean_path(dfhack.getHackPath()), clean_path(old_cwd .. '/hack/'))
+        expect.eq(clean_path(dfhack.getHackPath()), clean_path(fs.getcwd()))
+    end)
+end