Add initial warning for Termux users

The protoc binary on Maven Central may return 'Bad System Call'
for users on Termux using AARCH64 systems.

src/main/java/io/github/ascopes/protobufmavenplugin/platform/HostEnvironment.java

@@ -115,6 +115,15 @@ public static Set<String> systemPathExtensions() {
     }
   }
 
+  /**
+   * Get the current working directory.
+   *
+   * @return the current working directory.
+   */
+  public static Path workingDirectory() {
+    return Path.of("").toAbsolutePath();
+  }
+
   // Visible for testing purposes only.
   static Optional<String> environmentVariable(String name) {
     return ofNullable(System.getenv(name));

src/main/java/io/github/ascopes/protobufmavenplugin/resolve/MavenProtocCoordinateFactory.java

@@ -15,6 +15,8 @@
  */
 package io.github.ascopes.protobufmavenplugin.resolve;
 
+import java.nio.file.Files;
+import java.nio.file.Path;
 import io.github.ascopes.protobufmavenplugin.platform.HostEnvironment;
 import org.apache.maven.shared.transfer.artifact.ArtifactCoordinate;
 import org.apache.maven.shared.transfer.artifact.DefaultArtifactCoordinate;
@@ -43,6 +45,8 @@ public final class MavenProtocCoordinateFactory {
    * @throws ProtocResolutionException if the system is not supported.
    */
   public ArtifactCoordinate create(String versionRange) throws ProtocResolutionException {
+    emitPlatformWarnings();
+
     var coordinate = new DefaultArtifactCoordinate();
     coordinate.setGroupId(GROUP_ID);
     coordinate.setArtifactId(ARTIFACT_ID);
@@ -52,6 +56,17 @@ public ArtifactCoordinate create(String versionRange) throws ProtocResolutionExc
     return coordinate;
   }
 
+  private void emitPlatformWarnings() {
+    if (HostEnvironment.workingDirectory().toString().startsWith("/data/data/com.termux")) {
+      LOGGER.warn(
+          "It appears you are running on Termux. You may have difficulties "
+              + "invoking the 'protoc' executable from Maven Central. If this "
+              + "is an issue, install protoc directly ('pkg in protoc'), and "
+              + "invoke Maven with '-Dprotoc.version=PATH' instead."
+      );
+    }
+  }
+
   private String determineClassifier() throws ProtocResolutionException {
     String classifier;
 

src/test/java/io/github/ascopes/protobufmavenplugin/platform/HostEnvironmentTest.java

@@ -349,4 +349,12 @@ void environmentVariableReturnsEmptyOptionalWhenVariableIsNotSet() {
     assertThat(HostEnvironment.environmentVariable(unexpectedVariableName))
         .isEmpty();
   }
+
+  @DisplayName(".workingDirectory() returns the working directory")
+  @Test
+  void workingDirectoryReturnsTheWorkingDirectory() {
+    // Then
+    assertThat(HostEnvironment.workingDirectory())
+        .isEqualTo(Path.of("").toAbsolutePath());
+  }
 }