AP_Filesystem: added load_file() API

this makes it very easy to load a small file from sdcard or ROMFS

libraries/AP_Filesystem/AP_Filesystem.cpp

@@ -228,6 +228,16 @@ void AP_Filesystem::unmount(void)
     return LOCAL_BACKEND.fs.unmount();
 }
 
+/*
+  load a file to memory as a single chunk. Use only for small files
+ */
+FileData *AP_Filesystem::load_file(const char *filename)
+{
+    const Backend &backend = backend_by_path(filename);
+    return backend.fs.load_file(filename);
+}
+
+
 namespace AP
 {
 AP_Filesystem &FS()

libraries/AP_Filesystem/AP_Filesystem.h

@@ -89,6 +89,11 @@ public:
     // unmount filesystem for reboot
     void unmount(void);
 
+    /*
+      load a full file. Use delete to free the data
+     */
+    FileData *load_file(const char *filename);
+    
 private:
     struct Backend {
         const char *prefix;

libraries/AP_Filesystem/AP_Filesystem_ROMFS.cpp

@@ -212,4 +212,28 @@ bool AP_Filesystem_ROMFS::set_mtime(const char *filename, const uint32_t mtime_s
     return false;
 }
 
+/*
+  load a full file. Use delete to free the data
+  we override this in ROMFS to avoid taking twice the memory
+*/
+FileData *AP_Filesystem_ROMFS::load_file(const char *filename)
+{
+    FileData *fd = new FileData(this);
+    if (!fd) {
+        return nullptr;
+    }
+    fd->data = AP_ROMFS::find_decompress(filename, fd->length);
+    if (fd->data == nullptr) {
+        delete fd;
+        return nullptr;
+    }
+    return fd;
+}
+
+// unload data from load_file()
+void AP_Filesystem_ROMFS::unload_file(FileData *fd)
+{
+    AP_ROMFS::free(fd->data);
+}
+
 #endif // HAL_HAVE_AP_ROMFS_EMBEDDED_H

libraries/AP_Filesystem/AP_Filesystem_ROMFS.h

@@ -43,6 +43,14 @@ public:
     // set modification time on a file
     bool set_mtime(const char *filename, const uint32_t mtime_sec) override;
 
+    /*
+      load a full file. Use delete to free the data
+     */
+    FileData *load_file(const char *filename) override;
+
+    // unload data from load_file()
+    void unload_file(FileData *fd) override;
+    
 private:
     // only allow up to 4 files at a time
     static constexpr uint8_t max_open_file = 4;

libraries/AP_Filesystem/AP_Filesystem_backend.cpp

@@ -0,0 +1,73 @@
+/*
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "AP_Filesystem.h"
+
+/*
+  load a full file. Use delete to free the data
+*/
+FileData *AP_Filesystem_Backend::load_file(const char *filename)
+{
+    struct stat st;
+    if (stat(filename, &st) != 0) {
+        return nullptr;
+    }
+    FileData *fd = new FileData(this);
+    if (fd == nullptr) {
+        return nullptr;
+    }
+    void *data = malloc(st.st_size);
+    if (data == nullptr) {
+        delete fd;
+        return nullptr;
+    }
+    int d = open(filename, O_RDONLY);
+    if (d == -1) {
+        free(data);
+        delete fd;
+        return nullptr;
+    }
+    if (read(d, data, st.st_size) != st.st_size) {
+        close(d);
+        free(data);
+        delete fd;
+        return nullptr;
+    }
+    close(d);
+    fd->length = st.st_size;
+    fd->data = (const uint8_t *)data;
+    return fd;
+}
+
+/*
+  unload a FileData object
+*/
+void AP_Filesystem_Backend::unload_file(FileData *fd)
+{
+    if (fd->data != nullptr) {
+        free(const_cast<uint8_t *>(fd->data));
+        fd->data = nullptr;
+    }
+}
+
+/*
+  destructor for FileData
+ */
+FileData::~FileData()
+{
+    if (backend != nullptr) {
+        ((AP_Filesystem_Backend *)backend)->unload_file(this);
+    }
+}

libraries/AP_Filesystem/AP_Filesystem_backend.h

@@ -22,6 +22,21 @@
 
 #include "AP_Filesystem_Available.h"
 
+// returned structure from a load_file() call
+class FileData {
+public:
+    uint32_t length;
+    const uint8_t *data;
+
+    FileData(void *_backend) :
+        backend(_backend) {}
+    
+    // destructor to free data
+    ~FileData();
+private:
+    const void *backend;
+};
+
 class AP_Filesystem_Backend {
 
 public:
@@ -55,4 +70,12 @@ public:
 
     // unmount filesystem for reboot
     virtual void unmount(void) {}
+
+    /*
+      load a full file. Use delete to free the data
+     */
+    virtual FileData *load_file(const char *filename);
+
+    // unload data from load_file()
+    virtual void unload_file(FileData *fd);
 };