Basic version with example

This is not a complete solution but has the base form for how the
project is laid out and has an example showing how it can be used in a
project.

To test with telegraf you'll need to use the changes in this PR:
influxdata/telegraf#6024

.gitmodules

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+[submodule "example/telegraf-plugin-go-glue"]
+	path = example/telegraf-plugin-go-glue
+	url = https://github.com/StarryInternet/telegraf-plugin-go-glue
+[submodule "telegraf-plugin-go-glue"]
+	path = telegraf-plugin-go-glue
+	url = https://github.com/StarryInternet/telegraf-plugin-go-glue

.rustfmt.toml

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+# line length
+error_on_line_overflow = true
+max_width = 100
+
+# comments
+comment_width = 100
+wrap_comments = false # rustfmt is broken currently and doesn't softwrap
+normalize_comments = true
+
+# commas
+trailing_comma = "Never"
+match_block_trailing_comma = true
+
+# code
+use_try_shorthand = true
+binop_separator = "Back"
+format_strings = false # rustfmt string handling is horribly busted
+
+# imports
+reorder_imports = true

Cargo.toml

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+[package]
+name = "telegraf-plugin"
+version = "0.1.0"
+authors = ["Timidger <pcarpenter@starry.com>"]
+edition = "2018"
+build = "build.rs"
+
+[lib]
+proc-macro = true
+
+[build-dependencies]
+bindgen = "0.49.*"
+
+[dependencies]
+chrono = "0.4"
+libc = "0.2"
+syn = { version = "0.15", features = ["full", "fold"] }
+quote = "0.6"
+proc-macro2 = "0.4"

build.rs

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+// Copyright 2019. Starry, Inc. All Rights Reserved.
+fn main() {
+    let builder = bindgen::builder()
+        .derive_debug(true)
+        .derive_default(true)
+        .derive_copy(true)
+        .generate_comments(true)
+        .blacklist_function("sample_config")
+        .blacklist_function("description")
+        .blacklist_function("gather")
+        .header("telegraf-plugin-go-glue/c_api.h")
+        .ctypes_prefix("libc");
+    let generated = builder.generate().unwrap();
+    generated.write_to_file("src/gen.rs").unwrap();
+}

example/Cargo.toml

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+[package]
+name = "telegraf-plugin-example"
+version = "0.1.0"
+authors = ["Timidger <pcarpenter@starry.com>"]
+edition = "2018"
+
+[lib]
+crate_type = ["staticlib"]
+
+[dependencies]
+libc = "0.2"
+telegraf-plugin = { path = "../" }

example/makefile-template.toml

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+[tasks.rust_build]
+command = "cargo"
+args = ["build"]
+
+[tasks.copy_go_files]
+script = [
+'''
+cp target/debug/libtelegraf_plugin_example.a ./plugin.a
+cp telegraf-plugin-go-glue/lib.go ./
+cp telegraf-plugin-go-glue/c_api.h ./
+'''
+]
+
+[tasks.go_build]
+command = "go"
+args = ["build", "-buildmode=plugin", "-o=plugin.so"]
+dependencies = ["rust_build", "copy_go_files"]
+
+[tasks.cleanup]
+script = [
+'''
+rm plugin.a
+rm lib.go
+rm c_api.h
+if [ -f plugin.so ]; then
+    chmod a+x plugin.so
+fi
+'''
+]
+
+# This must run after the Rust build,
+# so that it can link the Rust artifact to it.
+[tasks.build]
+dependencies = [
+    "rust_build",
+	"go_build",
+	"cleanup"
+]
+
+[config]
+on_error_task = "cleanup"

example/src/lib.rs

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+// Copyright 2019. Starry, Inc. All Rights Reserved.
+use telegraf_plugin::link_to_go;
+
+macro_rules! map(
+    { $($key:expr => $value:expr),* } => {
+        {
+            #[allow(unused_mut)]
+            let mut m = ::std::collections::HashMap::new();
+            $(
+                m.insert($key, $value);
+            )*
+                m
+        }
+    };
+);
+
+#[link_to_go("Description of the plugin", "<sample_config>")]
+fn collect_metric() {
+    AddField(
+        "rust-metric".into(),
+        map! {"rust_key".into() => "rust_value".into()},
+        map! {"rust_field".into() => 100000000.into()},
+        None
+    )
+}

example/telegraf-plugin-go-glue

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+../telegraf-plugin-go-glue/