AP_Filesystem: list virtual @SYS, @MISSION etc directories in /

Adds virtual directory entries for these virtual filesystems in /

RTL> ftp list
RTL> Listing /
 D @MISSION
 D @PARAM
 D @ROMFS
 D @SYS
 D APM
 D log
   V5_BT.dfu	10541
   bootlog.txt	297
   dataman	350216
   message-intervals-chan0.txt	7
Total size 352.60 kByte
ftp list @MISSION
RTL> Listing @MISSION
LIST: OP seq:7 sess:2 opcode:129 req_opcode:3 size:2 bc:0 ofs:0 plen=2 [2]
ftp list @ROMFS
RTL> Listing @ROMFS
   bootloader.bin	16448
   hwdef.dat	5743
   io_firmware.bin	40880
Total size 61.59 kByte

This PR also makes us *much* more lenient in what we accept for looking at virtual filesystems, so

ftp list @SYS
ftp list /@SYS
ftp list @SYS/
ftp list /@SYS/

should all work

libraries/AP_Filesystem/AP_Filesystem.cpp

@@ -63,18 +63,16 @@ static AP_Filesystem_Mission fs_mission;
 const AP_Filesystem::Backend AP_Filesystem::backends[] = {
     { nullptr, fs_local },
 #if AP_FILESYSTEM_ROMFS_ENABLED
-    { "@ROMFS/", fs_romfs },
     { "@ROMFS", fs_romfs },
 #endif
 #if AP_FILESYSTEM_PARAM_ENABLED
-    { "@PARAM/", fs_param },
+    { "@PARAM", fs_param },
 #endif
 #if AP_FILESYSTEM_SYS_ENABLED
-    { "@SYS/", fs_sys },
     { "@SYS", fs_sys },
 #endif
 #if AP_FILESYSTEM_MISSION_ENABLED
-    { "@MISSION/", fs_mission },
+    { "@MISSION", fs_mission },
 #endif
 };
 
@@ -90,10 +88,19 @@ extern const AP_HAL::HAL& hal;
  */
 const AP_Filesystem::Backend &AP_Filesystem::backend_by_path(const char *&path) const
 {
+    // ignore leading slashes:
+    const char *path_with_no_leading_slash = path;
+    if (path_with_no_leading_slash[0] == '/') {
+        path_with_no_leading_slash = &path_with_no_leading_slash[1];
+    }
     for (uint8_t i=1; i<NUM_BACKENDS; i++) {
         const uint8_t plen = strlen(backends[i].prefix);
-        if (strncmp(path, backends[i].prefix, plen) == 0) {
+        if (strncmp(path_with_no_leading_slash, backends[i].prefix, plen) == 0) {
+            path = path_with_no_leading_slash;
             path += plen;
+            if (strlen(path) > 0 && path[0] == '/') {
+                path++;
+            }
             return backends[i];
         }
     }
@@ -188,6 +195,18 @@ int AP_Filesystem::rename(const char *oldpath, const char *newpath)
 
 AP_Filesystem::DirHandle *AP_Filesystem::opendir(const char *pathname)
 {
+    // support reading a list of "@" filesystems (e.g. @SYS) in
+    // listing of root directory.  Note that backend_by_path modifies
+    // its parameter.
+    if (strlen(pathname) == 0 ||
+        (strlen(pathname) == 1 && pathname[0] == '/')) {
+        virtual_dirent.backend_ofs = 0;
+        virtual_dirent.d_off = 0;
+        virtual_dirent.de.d_type = DT_DIR;
+    } else {
+        virtual_dirent.backend_ofs = 255;
+    }
+
     const Backend &backend = backend_by_path(pathname);
     DirHandle *h = new DirHandle;
     if (!h) {
@@ -199,6 +218,7 @@ AP_Filesystem::DirHandle *AP_Filesystem::opendir(const char *pathname)
         return nullptr;
     }
     h->fs_index = BACKEND_IDX(backend);
+
     return h;
 }
 
@@ -208,7 +228,26 @@ struct dirent *AP_Filesystem::readdir(DirHandle *dirp)
         return nullptr;
     }
     const Backend &backend = backends[dirp->fs_index];
-    return backend.fs.readdir(dirp->dir);
+    struct dirent * ret = backend.fs.readdir(dirp->dir);
+    if (ret != nullptr) {
+        return ret;
+    }
+
+    // virtual directory entries in the root directory (e.g. @SYS, @MISSION)
+    for (; ret == nullptr && virtual_dirent.backend_ofs < ARRAY_SIZE(AP_Filesystem::backends); virtual_dirent.backend_ofs++) {
+        const char *prefix = backends[virtual_dirent.backend_ofs].prefix;
+        if (prefix == nullptr) {
+            continue;
+        }
+        if (prefix[0] != '@') {
+            continue;
+        }
+        // found a virtual directory we haven't returned yet
+        strncpy_noterm(virtual_dirent.de.d_name, prefix, sizeof(virtual_dirent.de.d_name));
+        virtual_dirent.d_off++;
+        ret = &virtual_dirent.de;
+    }
+    return ret;
 }
 
 int AP_Filesystem::closedir(DirHandle *dirp)

libraries/AP_Filesystem/AP_Filesystem.h

@@ -155,6 +155,14 @@ private:
       find backend by open fd
      */
     const Backend &backend_by_fd(int &fd) const;
+
+    // support for listing out virtual directory entries (e.g. @SYS
+    // then @MISSION)
+    struct {
+        uint8_t backend_ofs;
+        struct dirent de;
+        uint8_t d_off;
+    } virtual_dirent;
 };
 
 namespace AP {