USB MSC Initiator: Allow disabling writes

USB MSC drive can be configured as readonly by setting in .ini:

[SCSI]
InitiatorMSC = 1 # Needed to enable MSC mode with SD card present
InitiatorMSCReadOnly = 1

src/ZuluSCSI_msc_initiator.cpp

@@ -75,6 +75,8 @@ static struct {
     bool prefetch_use_read10;
     bool prefetch_done; // True after prefetch is complete
 
+    bool readonly; // Disable writing to any drives
+
     // Periodic status reporting to log output
     uint32_t status_prev_time;
     uint32_t status_interval;
@@ -159,6 +161,12 @@ bool setup_msc_initiator()
     }
 
     g_msc_initiator_state.status_interval = ini_getl("SCSI", "InitiatorMSCStatusInterval", 5000, CONFIGFILE);
+    g_msc_initiator_state.readonly = ini_getbool("SCSI", "InitiatorMSCReadOnly", false, CONFIGFILE);
+
+    if (g_msc_initiator_state.readonly)
+    {
+        logmsg("--- Initiator is configured in read-only mode: writes to device are prevented");
+    }
 
     scsiInitiatorInit();
 
@@ -285,6 +293,11 @@ bool init_msc_is_writable_cb (uint8_t lun)
         return false;
     }
 
+    if (g_msc_initiator_state.readonly)
+    {
+        return false;
+    }
+
     LED_ON();
     g_msc_initiator_state.status_reqcount++;
 
@@ -545,6 +558,12 @@ int32_t init_msc_write10_cb(uint8_t lun, uint32_t lba, uint32_t offset, uint8_t
         return -1;
     }
 
+    if (g_msc_initiator_state.readonly)
+    {
+        logmsg("--- Refusing host write request, InitiatorMSCReadOnly is set.");
+        return -1;
+    }
+
     int status = -1;
 
     int target_id = get_target(lun);

zuluscsi.ini

@@ -48,6 +48,7 @@
 #InitiatorUseRead10 = 0 # 0: Always use READ6 command, 1: Always use READ10 command, not set: Autodetect READ10 support
 
 #InitiatorMSC = 0 # Force USB MSC mode for initiator. By default enabled only if SD card is not inserted.
+#InitiatorMSCReadOnly = 0 # Prevent writing to the drive through USB MSC
 #InitiatorMSCDisablePrefetch = 0 # Disable read prefetching in USB MSC mode
 #InitiatorMSCStatusInterval = 5000 # Periodically report access status to log