lint the json on update server startup

update-server/src/main/groovy/com/muwire/update/UpdateServer.groovy

@@ -1,5 +1,8 @@
 package com.muwire.update
 
+import net.i2p.data.Base64
+import net.i2p.util.VersionComparator
+
 import java.util.logging.Level
 
 import groovy.json.JsonSlurper
@@ -42,6 +45,9 @@ class UpdateServer {
             System.exit(1)
         }
 
+        byte [] json = loadJson(update)
+        log.info("update.json sanity check passed")
+        
         Properties props = System.getProperties().clone()
         props["inbound.nickname"] = "MuWire UpdateServer"
         def i2pPropsFile = new File(home, "i2p.properties")
@@ -51,19 +57,67 @@ class UpdateServer {
         session = i2pClient.createSession(new FileInputStream(keyFile), props)
         myDest = session.getMyDestination()
 
-        session.addMuxedSessionListener(new Listener(update), I2PSession.PROTO_DATAGRAM, I2PSession.PORT_ANY)
+        session.addMuxedSessionListener(new Listener(json), I2PSession.PROTO_DATAGRAM, I2PSession.PORT_ANY)
         session.connect()
         log.info("Connected, going to sleep")
         while(true)
             Thread.sleep(Integer.MAX_VALUE)
 
     }
+    
+    /** also performs sanity checks */
+    private static byte[] loadJson(File file) {
+        // 1. Check if valid json
+        JsonSlurper slurper = new JsonSlurper()
+        def parsed = slurper.parse(file)
+        
+        // 2. check if version parses
+        if (VersionComparator.comp("0.0.0", (String)parsed.version) >= 0)
+            throw new Exception("version invalid ${parsed.version}")
+        
+        // 3. check string fields
+        ["signer","text"].each { checkString(parsed[it])}
+        
+        // 4. check infohashes
+        ["infoHash","exe","mac","appimage","appimage-aarch64"].each {checkInfoHash(parsed[it])}
+        
+        // 5. check beta
+        if (parsed.beta != null) {
+            
+            // 1. Version must be an integer
+            Integer.parseInt(parsed.beta.version)
+            
+            // 2. text fields
+            ["signer","text"].each {checkString(parsed.beta[it])}
+            
+            // 3. infohashes
+            ["exe","appimage","mac"].each {
+                if (parsed.beta[it] != null)
+                    checkInfoHash(parsed.beta[it])
+            }
+        }
+        
+        // if we got here we're good
+        file.bytes
+    }
+    
+    private static void checkString(def obj) {
+        if (!(obj instanceof String))
+            throw new Exception("object not a string")
+    }
+    
+    private static void checkInfoHash(def obj) {
+        String s = (String) obj
+        byte [] decoded = Base64.decode(s)
+        if (decoded.length != 32)
+            throw new Exception("invalid infohash")
+    }
 
     static class Listener implements I2PSessionMuxedListener {
 
-        private final File json
+        private final byte[] json
         private final def slurper = new JsonSlurper()
-        Listener(File json) {
+        Listener(byte[] json) {
             this.json = json
         }
 
@@ -87,7 +141,7 @@ class UpdateServer {
                 log.info("Got an update ping from "+sender.toBase32() + " reported version "+payload?.myVersion)
 
                 def maker = new I2PDatagramMaker(session)
-                def response = maker.makeI2PDatagram(json.bytes)
+                def response = maker.makeI2PDatagram(json)
                 session.sendMessage(sender, response, I2PSession.PROTO_DATAGRAM, 0, 2)
             } catch (Exception e) {
                 log.log(Level.WARNING, "exception responding to update request",e)