bug #4184: fix config file loss due to filesystem write errors (#4397)

* Use SafeFile for atomic file writing (with xor checksum readback)
* Write db.proto last because it could be the largest file on the FS (and less critical)
* Don't keep a tmp file around while writing db.proto (because too big to fit two files in the filesystem)
* generate a new critial fault if we encounter errors writing to flash
either CriticalErrorCode_FLASH_CORRUPTION_RECOVERABLE or CriticalErrorCode_FLASH_CORRUPTION_UNRECOVERABLE
(depending on if the second write attempt worked)
* reformat the filesystem if we detect it is corrupted (then rewrite our config files) (only on nrf52 - not sure
yet if we should bother on ESP32)
* If we have to format the FS, make sure to preserve the oem.proto if it exists

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/SafeFile.cpp

@@ -0,0 +1,99 @@
+#include "SafeFile.h"
+
+#ifdef FSCom
+
+// Only way to work on both esp32 and nrf52
+static File openFile(const char *filename, bool fullAtomic)
+{
+    if (!fullAtomic)
+        FSCom.remove(filename); // Nuke the old file to make space (ignore if it !exists)
+
+    String filenameTmp = filename;
+    filenameTmp += ".tmp";
+
+    return FSCom.open(filenameTmp.c_str(), FILE_O_WRITE);
+}
+
+SafeFile::SafeFile(const char *_filename, bool fullAtomic)
+    : filename(_filename), f(openFile(_filename, fullAtomic)), fullAtomic(fullAtomic)
+{
+}
+
+size_t SafeFile::write(uint8_t ch)
+{
+    if (!f)
+        return 0;
+
+    hash ^= ch;
+    return f.write(ch);
+}
+
+size_t SafeFile::write(const uint8_t *buffer, size_t size)
+{
+    if (!f)
+        return 0;
+
+    for (size_t i = 0; i < size; i++) {
+        hash ^= buffer[i];
+    }
+    return f.write((uint8_t const *)buffer, size); // This nasty cast is _IMPORTANT_ otherwise the correct adafruit method does
+                                                   // not get used (they made a mistake in their typing)
+}
+
+/**
+ * Atomically close the file (deleting any old versions) and readback the contents to confirm the hash matches
+ *
+ * @return false for failure
+ */
+bool SafeFile::close()
+{
+    if (!f)
+        return false;
+
+    f.close();
+    if (!testReadback())
+        return false;
+
+    // brief window of risk here ;-)
+    if (fullAtomic && FSCom.exists(filename.c_str()) && !FSCom.remove(filename.c_str())) {
+        LOG_ERROR("Can't remove old pref file\n");
+        return false;
+    }
+
+    String filenameTmp = filename;
+    filenameTmp += ".tmp";
+    if (!renameFile(filenameTmp.c_str(), filename.c_str())) {
+        LOG_ERROR("Error: can't rename new pref file\n");
+        return false;
+    }
+
+    return true;
+}
+
+/// Read our (closed) tempfile back in and compare the hash
+bool SafeFile::testReadback()
+{
+    String filenameTmp = filename;
+    filenameTmp += ".tmp";
+    auto f2 = FSCom.open(filenameTmp.c_str(), FILE_O_READ);
+    if (!f2) {
+        LOG_ERROR("Can't open tmp file for readback\n");
+        return false;
+    }
+
+    int c = 0;
+    uint8_t test_hash = 0;
+    while ((c = f2.read()) >= 0) {
+        test_hash ^= (uint8_t)c;
+    }
+    f2.close();
+
+    if (test_hash != hash) {
+        LOG_ERROR("Readback failed hash mismatch\n");
+        return false;
+    }
+
+    return true;
+}
+
+#endif