fix(efs): place mount targets in their real subnet

CreateMountTarget now looks the subnet up in EC2 and takes the mount target's
VPC, Availability Zone and IP address from it, instead of reporting
vpc-12345678 and the region's first zone for every mount target. An unknown
subnet is refused with SubnetNotFound, and, as in AWS, a file system gets one
mount target per zone, all in one VPC (MountTargetConflict otherwise).

Lambda checks a function's VPC against its file system's mount targets, so the
made-up VPC kept consoles from finding the security group to use.

src/main/java/io/github/hectorvent/floci/services/efs/EfsException.java

@@ -29,6 +29,10 @@ public class EfsException extends AwsException {
         return new EfsException(Status.NOT_FOUND, "MountTargetNotFound", "Mount target " + mountTargetId + " does not exist.");
     }
     
+    public static EfsException subnetNotFound(String subnetId) {
+        return new EfsException(Status.BAD_REQUEST, "SubnetNotFound", "The subnet ID '" + subnetId + "' does not exist");
+    }
+
     public static EfsException mountTargetConflict(String message) {
         return new EfsException(Status.CONFLICT, "MountTargetConflict", message);
     }

src/main/java/io/github/hectorvent/floci/services/efs/EfsService.java

@@ -2,7 +2,10 @@ package io.github.hectorvent.floci.services.efs;
 
 import io.github.hectorvent.floci.core.storage.StorageBackend;
 import io.github.hectorvent.floci.core.storage.StorageFactory;
+import io.github.hectorvent.floci.core.common.AwsException;
 import io.github.hectorvent.floci.core.common.Resettable;
+import io.github.hectorvent.floci.services.ec2.Ec2Service;
+import io.github.hectorvent.floci.services.ec2.model.Subnet;
 import io.github.hectorvent.floci.services.efs.model.*;
 import jakarta.enterprise.context.ApplicationScoped;
 import jakarta.inject.Inject;
@@ -33,13 +36,16 @@ public class EfsService implements Resettable {
     private final ConcurrentHashMap<String, Object> syncLocks = new ConcurrentHashMap<>();
     private final RegionResolver regionResolver;
 
+    private final Ec2Service ec2Service;
+
     private Object lockFor(String key) {
         return syncLocks.computeIfAbsent(key, k -> new Object());
     }
 
     @Inject
-    public EfsService(StorageFactory storageFactory, RegionResolver regionResolver) {
+    public EfsService(StorageFactory storageFactory, RegionResolver regionResolver, Ec2Service ec2Service) {
         this.regionResolver = regionResolver;
+        this.ec2Service = ec2Service;
         this.fileSystemStore = storageFactory.create("efs", "efs-filesystems.json",
                 new com.fasterxml.jackson.core.type.TypeReference<java.util.Map<String, FileSystem>>() {});
         this.mountTargetStore = storageFactory.create("efs", "efs-mounttargets.json",
@@ -361,14 +367,24 @@ public class EfsService implements Resettable {
     public MountTarget createMountTarget(CreateMountTargetRequest request, String region) {
         synchronized (lockFor(regionKey(region, request.getFileSystemId()))) {
             FileSystem fs = getFileSystem(region, request.getFileSystemId());
+            Subnet subnet = requireSubnet(region, request.getSubnetId());
 
-            for (MountTarget existing : mountTargetStore.scan(k -> k.startsWith(region + "::"))) {
+            List<MountTarget> regionMountTargets = mountTargetStore.scan(k -> k.startsWith(region + "::"));
+            for (MountTarget existing : regionMountTargets) {
                 if (existing.getFileSystemId().equals(request.getFileSystemId())) {
                     if (existing.getSubnetId().equals(request.getSubnetId())) {
                         throw EfsException.mountTargetConflict("Mount target already exists in this subnet");
                     }
+                    // AWS allows one mount target per Availability Zone, all in one VPC.
+                    if (subnet.getAvailabilityZone().equals(existing.getAvailabilityZoneName())) {
+                        throw EfsException.mountTargetConflict("Mount target already exists in this AZ");
+                    }
+                    if (!subnet.getVpcId().equals(existing.getVpcId())) {
+                        throw EfsException.mountTargetConflict("Mount targets of a file system must be in the same VPC");
                     }
                 }
+            }
+            long inSubnet = regionMountTargets.stream().filter(existing -> request.getSubnetId().equals(existing.getSubnetId())).count();
             
             String mtId = "fsmt-" + UUID.randomUUID().toString().replace("-", "").substring(0, 17);
         
@@ -376,11 +392,11 @@ public class EfsService implements Resettable {
             mt.setMountTargetId(mtId);
             mt.setFileSystemId(request.getFileSystemId());
             mt.setSubnetId(request.getSubnetId());
-            mt.setIpAddress(request.getIpAddress() != null ? request.getIpAddress() : "10.0.0.10");
+            mt.setIpAddress(request.getIpAddress() != null ? request.getIpAddress() : hostAddress(subnet.getCidrBlock(), inSubnet));
             mt.setLifeCycleState(LifeCycleState.available);
-            mt.setVpcId("vpc-12345678");
+            mt.setVpcId(subnet.getVpcId());
-            mt.setAvailabilityZoneName(region + "a");
+            mt.setAvailabilityZoneName(subnet.getAvailabilityZone());
-            mt.setAvailabilityZoneId(generateAzId(region + "a"));
+            mt.setAvailabilityZoneId(generateAzId(subnet.getAvailabilityZone()));
         mt.setNetworkInterfaceId("eni-" + UUID.randomUUID().toString().replace("-", "").substring(0, 17));
         if (request.getSecurityGroups() != null) {
             mt.setSecurityGroups(new ArrayList<>(request.getSecurityGroups()));
@@ -613,6 +629,32 @@ public class EfsService implements Resettable {
         return region + "::" + id;
     }
 
+    private Subnet requireSubnet(String region, String subnetId) {
+        if (subnetId == null || subnetId.isBlank()) {
+            throw EfsException.badRequest("SubnetId is required.");
+        }
+        try {
+            return ec2Service.requireSubnet(region, subnetId);
+        } catch (AwsException e) {
+            throw EfsException.subnetNotFound(subnetId);
+        }
+    }
+
+    /**
+     * An address inside the subnet for its n-th mount target, skipping the first four
+     * addresses AWS reserves in every subnet.
+     */
+    static String hostAddress(String cidrBlock, long index) {
+        String[] parts = cidrBlock.split("/");
+        String[] octets = parts[0].split("\\.");
+        long base = 0;
+        for (String octet : octets) {
+            base = (base << 8) | Long.parseLong(octet);
+        }
+        long address = base + 4 + index;
+        return ((address >> 24) & 0xff) + "." + ((address >> 16) & 0xff) + "." + ((address >> 8) & 0xff) + "." + (address & 0xff);
+    }
+
     private String generateAzId(String azName) {
         if (azName == null || azName.length() < 2) return azName + "-id";
         String region = azName.substring(0, azName.length() - 1);

src/test/java/io/github/hectorvent/floci/services/efs/EfsIntegrationTest.java

@@ -90,7 +90,7 @@ class EfsIntegrationTest {
             .body("""
                 {
                     "FileSystemId": "%s",
-                    "SubnetId": "subnet-12345",
+                    "SubnetId": "subnet-default-us-east-1-a",
                     "SecurityGroups": ["sg-11111"]
                 }
                 """.formatted(fileSystemId))
@@ -100,7 +100,10 @@ class EfsIntegrationTest {
             .statusCode(200)
             .body("MountTargetId", startsWith("fsmt-"))
             .body("FileSystemId", equalTo(fileSystemId))
-            .body("SubnetId", equalTo("subnet-12345"))
+            .body("SubnetId", equalTo("subnet-default-us-east-1-a"))
+            .body("VpcId", equalTo("vpc-default-us-east-1"))
+            .body("AvailabilityZoneName", equalTo("us-east-1a"))
+            .body("IpAddress", equalTo("172.31.0.4"))
             .extract().jsonPath().getString("MountTargetId");
     }
 

src/test/java/io/github/hectorvent/floci/services/efs/EfsMountTargetSubnetTest.java

@@ -0,0 +1,91 @@
+package io.github.hectorvent.floci.services.efs;
+
+import io.github.hectorvent.floci.testing.RestAssuredJsonUtils;
+import io.quarkus.test.junit.QuarkusTest;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import static io.restassured.RestAssured.given;
+import static org.hamcrest.Matchers.equalTo;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/** Mount targets take their VPC, zone and address from the EC2 subnet they are created in. */
+@QuarkusTest
+class EfsMountTargetSubnetTest {
+
+    @BeforeAll
+    static void configureRestAssured() {
+        RestAssuredJsonUtils.configureAwsContentTypes();
+    }
+
+    private String createFileSystem(String token) {
+        return given()
+            .contentType("application/json")
+            .body("{\"CreationToken\": \"" + token + "\"}")
+        .when()
+            .post("/2015-02-01/file-systems")
+        .then()
+            .statusCode(201)
+            .extract().jsonPath().getString("FileSystemId");
+    }
+
+    private io.restassured.response.ValidatableResponse createMountTarget(String fileSystemId, String subnetId) {
+        return given()
+            .contentType("application/json")
+            .body("{\"FileSystemId\": \"" + fileSystemId + "\", \"SubnetId\": \"" + subnetId + "\"}")
+        .when()
+            .post("/2015-02-01/mount-targets")
+        .then();
+    }
+
+    @Test
+    void usesTheZoneAndVpcOfEachSubnet() {
+        String fileSystemId = createFileSystem("subnet-zones");
+
+        createMountTarget(fileSystemId, "subnet-default-us-east-1-b")
+            .statusCode(200)
+            .body("VpcId", equalTo("vpc-default-us-east-1"))
+            .body("AvailabilityZoneName", equalTo("us-east-1b"))
+            .body("IpAddress", equalTo("172.31.16.4"));
+        createMountTarget(fileSystemId, "subnet-default-us-east-1-c")
+            .statusCode(200)
+            .body("AvailabilityZoneName", equalTo("us-east-1c"));
+    }
+
+    @Test
+    void refusesASubnetThatDoesNotExist() {
+        String fileSystemId = createFileSystem("subnet-missing");
+
+        createMountTarget(fileSystemId, "subnet-nope")
+            .statusCode(400)
+            .body("__type", equalTo("SubnetNotFound"));
+    }
+
+    @Test
+    void allowsOneMountTargetPerZone() {
+        String fileSystemId = createFileSystem("subnet-one-per-zone");
+        String vpcId = "vpc-default-us-east-1";
+        String secondSubnet = given()
+            .formParam("Action", "CreateSubnet")
+            .formParam("VpcId", vpcId)
+            .formParam("CidrBlock", "172.31.200.0/24")
+            .formParam("AvailabilityZone", "us-east-1a")
+            .header("Authorization", "AWS4-HMAC-SHA256 Credential=test/20260101/us-east-1/ec2/aws4_request")
+        .when()
+            .post("/")
+        .then()
+            .statusCode(200)
+            .extract().path("CreateSubnetResponse.subnet.subnetId");
+
+        createMountTarget(fileSystemId, "subnet-default-us-east-1-a").statusCode(200);
+        createMountTarget(fileSystemId, secondSubnet)
+            .statusCode(409)
+            .body("__type", equalTo("MountTargetConflict"));
+    }
+
+    @Test
+    void picksAddressesAfterTheFourAwsReserves() {
+        assertEquals("10.0.1.4", EfsService.hostAddress("10.0.1.0/24", 0));
+        assertEquals("10.0.2.1", EfsService.hostAddress("10.0.1.0/24", 253));
+    }
+}